Forgive me for the overly detailed title. The challenge is easy enough to understand, challenging without being too restrictive, and actually quite fun (if you like Poison-types, that is).
The challenge plays as a regular Nuzlocke, with three additional rules:
1. You may catch the first Pokémon on a route only if it is a Poison-type.
This may sound extremely restrictive, but it's not! Poison-types are not only the most numerous type in the first generation, but are also found on nearly every route, with only a few exceptions. Most Poison-types can be found on several different routes, giving you many chances to obtain them.
2. Items are not allowed to be purchased in marts.
This rule excludes the vending machines on top of the Celadon Department Store for obvious reasons, and the Game Corner in the same city for reasons that will be important later! Items like Potions and especially Poké Balls will have to be conserved carefully.
3. Any Pokémon that can learn a HM move, must learn that move as soon as the HM in question is obtained, or the Pokémon is captured or evolved.
Simply another flavour rule, though only Cut is a real restriction here, and a very minor one at that.
Now, in rough order of appearance, the Pokémon:
The Starter
Bulbasaur is the only choice if you want to do a true Monotype run of Red or Blue. Charmander spends two stages as a pure Fire-type before it becomes the Flying-type that would otherwise be an option, whereas Squirtle is a curious case. One would think that Water is the obvious choice, but Cut throws a wrench into your plans! As it turns out, the only two Water-types that learn the move, Tentacool and Krabby, can only be obtained after you have used the move Cut. This leaves us with Poison as the only option (Grass and Fire were never options), and it is suitable for the task. As is commonly known, Bulbasaur has a very good match-up against the early gyms, but is less efficient when it comes to clearing routes. It also, and this is important, learns Cut, making sure that you will never get stuck behind a bush as long as your starter is alive.
Suggested goal moveset: Razor Leaf, Sleep Powder, Leech Seed, CUT
Nidoran
In Red, this will most likely eventually be a Nidoking, in Blue, a Nidoqueen, but this does not matter in the big picture as they have similar qualities. Known as one of the most powerful Pokémon for any in-game run, this is no exception, except capturing it on Route 22 is by no means guaranteed. Their combined 45% encounter probability is further compounded by the fact that you'll want to cross the grass of Route 22 twice before capturing anything. If you do get it, consider yourself lucky, as Bulbasaur will have gotten the best possible companion for a good part of the game! If not, there will be opportunities later. The Safari Zone gives you another very decent chance to catch it, not even spending a Poké Ball from your inventory to do so. If all else fails, you can even buy a Nidorina (R) or Nidorino (B) from the Celadon Game Corner, so there's nothing keeping you from getting both Nidoking and Nidoqueen, even if you only need one. They both learn both Strength and Surf, which are not only required to progress past certain points in the game, but also work very well together. Allow me for the moment to digress, and sing the praises of the humble StrengthSurfer! These are two decently strong moves, with 15 PP and 99.6% accuracy, one special and the other physical, with perfect neutral coverage between them.
Suggested goal moveset: Earthquake, Ice Beam, SURF, STRENGTH
The Not-So-Greats
I have elected to put Weedle and Zubat in the same category, because there is not so much to say about them. They can, however, help fill up your team in the early game. In Blue, you're unlikely to capture Weedle, but in Red, there's a combined 85% chance to find either Weedle or Kakuna as your first encounter in Viridian Forest, with a smaller second chance on Route 2 just outside. Zubat is comparable to this, with 79% in Mt. Moon, and unlike Weedle, this probability is the same in both versions. Beedrill can also learn Cut, not to mention that Twineedle has situational worth, and Golbat can use Confuse Ray and switch into a Dig aimed at Ground-weak teammembers, but that's about as far as their usefulness goes.
Suggested goal movesets: Don't bother
The Version Exclusives
Somewhere in the early game after Mt. Moon, each game allows you to capture a Wrap user, and a Grass-type which learns Sleep Powder. The difference is that these are two different Pokémon in Red, but only one in Blue. Ekans, Oddish and Bellsprout can all be found on a number of routes throughout the early and middle game, making their capture relatively likely. As far as their usefulness goes, their key attributes are mentioned in the first sentence. Ekans gets Wrap, Oddish gets Sleep Powder, and Bellsprout gets both. Ekans can also be given the Dig TM early on, which has good coverage along with at first Bite, and later on the now mandatory Strength. Meanwhile the Grass-types suffer from not being able to get their hands (do they even have hands?) on a Leaf Stone. Nevertheless, Gloom and especially Weepinbell are far from worthless in this format, and can excel as a front-line party member with their early access to Sleep Powder. Weepinbell is also guaranteed to learn its best attacking move, Razor Leaf, which it can't learn as Victreebel.
Suggested goal movesets: Wrap/Glare/Dig/STRENGTH, Sleep Powder/Body Slam/Mega Drain/CUT, Wrap/Sleep Powder/Razor Leaf/CUT
The Ghost
With a 90% encounter percentage in Pokémon Tower, Gastly is a near-guaranteed encounter, and a great one at that. Even without the trade, Haunter is still quite fast, and has a good Special stat, too, but that's not the best. The Ghost typing is a big deal in Gen 1, where so many things rely on Normal-type moves, and a good few on Normal and Fighting. Gastly's (or Haunter's, as you will evolve it very soon after capture) typing neutralizes so many trainers, and even the odd Gym leader. If you can also spare the right TMs, Haunter is truly a force to be reckoned with.
Suggested goal moveset: Thunderbolt, Psychic, Confuse Ray, Hypnosis
The Moth
Most of what is said about Oddish and Gloom also applies to Venonat and Venomoth. They don't learn Cut, but you should have enough Cut users at this point anyway. They do, however, learn Psybeam and Psychic by level-up, though these moves come late, as do Stun Spore and Sleep Powder, so you're stuck with unimpressive moves for a good while.
Suggested goal moveset: You have more important things to do
The Jellyfish
Along with Bulbasaur, Nidoran and Gastly, Tentacool is the final piece of the Big Four in this challenge. This is a guaranteed encounter on every water route, though if you for some reason should be unable to catch one, or without a Surf user, you can also fish for it with the Super Rod. Great speed, an outstanding Special stat and a useful movepool gives you the tools needed to succeed with this Pokémon. That said, half of the movepool will be taken up by Cut and Surf. Though Surf is a great move for Tentacool/Tentacruel, the lack of choice will leave you with the following moveset in most cases, unless Tentacool was caught at such a high level it doesn't know Wrap.
Suggested goal moveset: Wrap, Blizzard, SURF, CUT
The Nasty Thing from Cinnabar
Finally, there's Koffing and Grimer. In the entrance area of Pokémon Mansion, these two and their evolved forms have a combined 50% chance to be the first to appear; essentially a coin flip. Out of these, it is Koffing (R) and Grimer (B) that have the highest probability of appearing, at 40%. If you don't get one of them, it's not a big deal, but they can each be nice to have. Their movepools are wide and unburdened by HM moves, so give them whatever TMs you have left and think might fit them.
Suggested goal moveset: Anything but Explosion (Sludge and Fire Blast seem like good options)
That was a quick overview of the available Pokémon. Now, to some of the possible problems that could arise with this challenge:
1. Not enough balls
I would advise you to go for the optional battle with the rival on Route 22 as soon as Bulbasaur is strong enough to beat him. That way, you can get five Poké Balls from Professor Oak afterwards, though this risks wasting the wild Pokémon encounter on Route 22, which might be the important Nidoran. The alternative is to go for the single Poké Ball in Viridian Forest, but this leaves you with few available balls for the rest of the game. Two important hidden items are the Great Balls on Route 4 and in the garbage can on SS Anne. Make sure you find them!
2. Wrong encounters, failed captures, small teams
With the restrictions in place, you run the risk of encountering Spearow on Route 22, Pikachu in Viridian Forest, Pidgey on Route 2, Geodude in Mt. Moon and so on. Do not despair! The probability of not encountering a single Poison-type on all the routes together is one I haven't calculated, but it ought to be exceedingly low. In the absolute worst case scenario, you can still beat the game with your starter and the Nido you purchased in Celadon.
3. No Fly, no Flash
As sad it is to see that no Poison-type supports Flash in this generation, you'll only ever use it once, in Rock Tunnel. Being without Fly is also an inconvenience, but that is all there is to it. With Diglett's Cave and the open gates around Saffron, you can get wherever you want to go in no time. The essential HM moves, Cut, Surf and Strength, are all covered several times over. Flash and Fly are only the luxuries that makes life easier in Kanto!
4. Sabrina
The elephant in the room. What are you going to do when each and every one of your Pokémon is weak to Alakazam's attacks? Luckily, there are a few ways around this. The easy answer is to overlevel your Pokémon to the point where it can outspeed and OHKO Sabrina's team. Nidoking and Nidoqueen are possible candidates for this, with STAB Earthquake, but also Beedrill, with a super effective Twineedle. Wrap is also an option, best suited for Tentacruel, who may have both the speed required to outspeed Alakazam as well as the Special stat to take a hit, should Wrap miss. Venusaur learns Sleep Powder at level 55, at which point it should be able to deal with Alakazam just fine. To a lesser degree, these problems also apply to your rival's Kadabra/Alakazam throughout the game.
So there! Go out and try this challenge! For added fun, have your rival do the same challenge with Flying-types, and watch him bend the rules picking Charmander, follow them with Pidgey as long as they're near Pallet, and then disregard them completely as soon as he's far enough away from his grandfather. Enjoy!
The challenge plays as a regular Nuzlocke, with three additional rules:
1. You may catch the first Pokémon on a route only if it is a Poison-type.
This may sound extremely restrictive, but it's not! Poison-types are not only the most numerous type in the first generation, but are also found on nearly every route, with only a few exceptions. Most Poison-types can be found on several different routes, giving you many chances to obtain them.
2. Items are not allowed to be purchased in marts.
This rule excludes the vending machines on top of the Celadon Department Store for obvious reasons, and the Game Corner in the same city for reasons that will be important later! Items like Potions and especially Poké Balls will have to be conserved carefully.
3. Any Pokémon that can learn a HM move, must learn that move as soon as the HM in question is obtained, or the Pokémon is captured or evolved.
Simply another flavour rule, though only Cut is a real restriction here, and a very minor one at that.
Now, in rough order of appearance, the Pokémon:
The Starter
Bulbasaur is the only choice if you want to do a true Monotype run of Red or Blue. Charmander spends two stages as a pure Fire-type before it becomes the Flying-type that would otherwise be an option, whereas Squirtle is a curious case. One would think that Water is the obvious choice, but Cut throws a wrench into your plans! As it turns out, the only two Water-types that learn the move, Tentacool and Krabby, can only be obtained after you have used the move Cut. This leaves us with Poison as the only option (Grass and Fire were never options), and it is suitable for the task. As is commonly known, Bulbasaur has a very good match-up against the early gyms, but is less efficient when it comes to clearing routes. It also, and this is important, learns Cut, making sure that you will never get stuck behind a bush as long as your starter is alive.
Suggested goal moveset: Razor Leaf, Sleep Powder, Leech Seed, CUT
Nidoran
In Red, this will most likely eventually be a Nidoking, in Blue, a Nidoqueen, but this does not matter in the big picture as they have similar qualities. Known as one of the most powerful Pokémon for any in-game run, this is no exception, except capturing it on Route 22 is by no means guaranteed. Their combined 45% encounter probability is further compounded by the fact that you'll want to cross the grass of Route 22 twice before capturing anything. If you do get it, consider yourself lucky, as Bulbasaur will have gotten the best possible companion for a good part of the game! If not, there will be opportunities later. The Safari Zone gives you another very decent chance to catch it, not even spending a Poké Ball from your inventory to do so. If all else fails, you can even buy a Nidorina (R) or Nidorino (B) from the Celadon Game Corner, so there's nothing keeping you from getting both Nidoking and Nidoqueen, even if you only need one. They both learn both Strength and Surf, which are not only required to progress past certain points in the game, but also work very well together. Allow me for the moment to digress, and sing the praises of the humble StrengthSurfer! These are two decently strong moves, with 15 PP and 99.6% accuracy, one special and the other physical, with perfect neutral coverage between them.
Suggested goal moveset: Earthquake, Ice Beam, SURF, STRENGTH
The Not-So-Greats
I have elected to put Weedle and Zubat in the same category, because there is not so much to say about them. They can, however, help fill up your team in the early game. In Blue, you're unlikely to capture Weedle, but in Red, there's a combined 85% chance to find either Weedle or Kakuna as your first encounter in Viridian Forest, with a smaller second chance on Route 2 just outside. Zubat is comparable to this, with 79% in Mt. Moon, and unlike Weedle, this probability is the same in both versions. Beedrill can also learn Cut, not to mention that Twineedle has situational worth, and Golbat can use Confuse Ray and switch into a Dig aimed at Ground-weak teammembers, but that's about as far as their usefulness goes.
Suggested goal movesets: Don't bother
The Version Exclusives
Somewhere in the early game after Mt. Moon, each game allows you to capture a Wrap user, and a Grass-type which learns Sleep Powder. The difference is that these are two different Pokémon in Red, but only one in Blue. Ekans, Oddish and Bellsprout can all be found on a number of routes throughout the early and middle game, making their capture relatively likely. As far as their usefulness goes, their key attributes are mentioned in the first sentence. Ekans gets Wrap, Oddish gets Sleep Powder, and Bellsprout gets both. Ekans can also be given the Dig TM early on, which has good coverage along with at first Bite, and later on the now mandatory Strength. Meanwhile the Grass-types suffer from not being able to get their hands (do they even have hands?) on a Leaf Stone. Nevertheless, Gloom and especially Weepinbell are far from worthless in this format, and can excel as a front-line party member with their early access to Sleep Powder. Weepinbell is also guaranteed to learn its best attacking move, Razor Leaf, which it can't learn as Victreebel.
Suggested goal movesets: Wrap/Glare/Dig/STRENGTH, Sleep Powder/Body Slam/Mega Drain/CUT, Wrap/Sleep Powder/Razor Leaf/CUT
The Ghost
With a 90% encounter percentage in Pokémon Tower, Gastly is a near-guaranteed encounter, and a great one at that. Even without the trade, Haunter is still quite fast, and has a good Special stat, too, but that's not the best. The Ghost typing is a big deal in Gen 1, where so many things rely on Normal-type moves, and a good few on Normal and Fighting. Gastly's (or Haunter's, as you will evolve it very soon after capture) typing neutralizes so many trainers, and even the odd Gym leader. If you can also spare the right TMs, Haunter is truly a force to be reckoned with.
Suggested goal moveset: Thunderbolt, Psychic, Confuse Ray, Hypnosis
The Moth
Most of what is said about Oddish and Gloom also applies to Venonat and Venomoth. They don't learn Cut, but you should have enough Cut users at this point anyway. They do, however, learn Psybeam and Psychic by level-up, though these moves come late, as do Stun Spore and Sleep Powder, so you're stuck with unimpressive moves for a good while.
Suggested goal moveset: You have more important things to do
The Jellyfish
Along with Bulbasaur, Nidoran and Gastly, Tentacool is the final piece of the Big Four in this challenge. This is a guaranteed encounter on every water route, though if you for some reason should be unable to catch one, or without a Surf user, you can also fish for it with the Super Rod. Great speed, an outstanding Special stat and a useful movepool gives you the tools needed to succeed with this Pokémon. That said, half of the movepool will be taken up by Cut and Surf. Though Surf is a great move for Tentacool/Tentacruel, the lack of choice will leave you with the following moveset in most cases, unless Tentacool was caught at such a high level it doesn't know Wrap.
Suggested goal moveset: Wrap, Blizzard, SURF, CUT
The Nasty Thing from Cinnabar
Finally, there's Koffing and Grimer. In the entrance area of Pokémon Mansion, these two and their evolved forms have a combined 50% chance to be the first to appear; essentially a coin flip. Out of these, it is Koffing (R) and Grimer (B) that have the highest probability of appearing, at 40%. If you don't get one of them, it's not a big deal, but they can each be nice to have. Their movepools are wide and unburdened by HM moves, so give them whatever TMs you have left and think might fit them.
Suggested goal moveset: Anything but Explosion (Sludge and Fire Blast seem like good options)
That was a quick overview of the available Pokémon. Now, to some of the possible problems that could arise with this challenge:
1. Not enough balls
I would advise you to go for the optional battle with the rival on Route 22 as soon as Bulbasaur is strong enough to beat him. That way, you can get five Poké Balls from Professor Oak afterwards, though this risks wasting the wild Pokémon encounter on Route 22, which might be the important Nidoran. The alternative is to go for the single Poké Ball in Viridian Forest, but this leaves you with few available balls for the rest of the game. Two important hidden items are the Great Balls on Route 4 and in the garbage can on SS Anne. Make sure you find them!
2. Wrong encounters, failed captures, small teams
With the restrictions in place, you run the risk of encountering Spearow on Route 22, Pikachu in Viridian Forest, Pidgey on Route 2, Geodude in Mt. Moon and so on. Do not despair! The probability of not encountering a single Poison-type on all the routes together is one I haven't calculated, but it ought to be exceedingly low. In the absolute worst case scenario, you can still beat the game with your starter and the Nido you purchased in Celadon.
3. No Fly, no Flash
As sad it is to see that no Poison-type supports Flash in this generation, you'll only ever use it once, in Rock Tunnel. Being without Fly is also an inconvenience, but that is all there is to it. With Diglett's Cave and the open gates around Saffron, you can get wherever you want to go in no time. The essential HM moves, Cut, Surf and Strength, are all covered several times over. Flash and Fly are only the luxuries that makes life easier in Kanto!
4. Sabrina
The elephant in the room. What are you going to do when each and every one of your Pokémon is weak to Alakazam's attacks? Luckily, there are a few ways around this. The easy answer is to overlevel your Pokémon to the point where it can outspeed and OHKO Sabrina's team. Nidoking and Nidoqueen are possible candidates for this, with STAB Earthquake, but also Beedrill, with a super effective Twineedle. Wrap is also an option, best suited for Tentacruel, who may have both the speed required to outspeed Alakazam as well as the Special stat to take a hit, should Wrap miss. Venusaur learns Sleep Powder at level 55, at which point it should be able to deal with Alakazam just fine. To a lesser degree, these problems also apply to your rival's Kadabra/Alakazam throughout the game.
So there! Go out and try this challenge! For added fun, have your rival do the same challenge with Flying-types, and watch him bend the rules picking Charmander, follow them with Pidgey as long as they're near Pallet, and then disregard them completely as soon as he's far enough away from his grandfather. Enjoy!