Being a Poison-type is like being dropped on your head at birth in RBY, so let's riff on Weezing.
I don't know why Weezing was made when it literally breathed to be outclassed by Muk in RBY. It feels like it came a generation too early; I can see it being a Steel-type if it was made in Gen 2. However, even there Magneton rightfully shoved it to the wall to take that privilege and say "it's just a prank bro".
"So what's with the Muk thing, PvK?!", I hear you scream from the rooftops. Well, it's true. The only real moves Weezing has over Muk are Haze and Smokescreen, while Muk gets Acid Armor (!), Mega Drain, and Body Slam, as well as a few other nifty tricks like Minimize. Weezing is meant to be a bulkier version, but when Muk gets the best Defense-boosting move in the game...well, you get the idea. Couple that in with the fact you get both in the same place - Grimer sooner in Yellow - and you have a recipe for "why the fuck would you use it then?". So you have two Pokemon with the best Poison STAB in Sludge, and one may as well not exist. That's pretty fucking dire.
It's nice that they became more distinguished in later generations, though. Weezing really needed Levitate and the Haze buffs to truly come into its own come ADV. It's come super far since then, too!