What effect did Pokemon have on the metagame?
Zapdos has a very strong presence in National Dex OU, both defensively and offensively. With access to exceptional coverage, recovery, and decent typing and bulk it fits on a plethora of team compositions and archetypes making it extremely flexible and versatile. It checks Kartana, Mega Loppuny, Corviknight, Mega Gyarados, Mega Scizor, and Rillaboom consistently, which most if not all can be extremely threatening if not accounted for in a team.
In what main roles was Pokemon used?
Zapdos is mostly seen on teams as a defensive pivot, but also has decent usage as a rain abuser and wallbreaker. For the most part, Zapdos has a role in checking physical attackers such as Mega Scizor, Kartana, Mega Gyarados, Rillaboom, and Mega Lopunny; as well as potentially beating many walls like Toxapex, Tapu Fini, Ferrothorn, Landorus-Therian, and Gliscor with its remarkable coverage in Heat Wave and Hidden Power Ice. As a rain abuser, Zapdos' coverage is nearly impossible to resist because of Weather Ball, and the 100% accurate Hurricane and Thunder. As of recent, Flyinium Z and SubRoost sets on Zapdos have been becoming more and more popular due to the ability to break and Pressure stall roadblocks such as Gastrodon, Ferrothorn, Clefable. Landorus-Therian, Garchomp, Kyurem, Heatran and Hippowdon after prior chip damage. Zapdos can take advantage of learning Defog although it loses to many Stealth Rock setters such as Garchomp, Mega Diancie, Landorus-Therian and Mega Tyranitar meaning other hazard removers outclass it. The reason Zapdos is better in rain over an alternative such as Thundurus-Therian is Flying-type coverage and it provides rain teams with a buffer against Kartana and Rillaboom which Thundurus-T just can't provide due to lesser defenses and lack of Static. Zapdos is also one of the better physically defensive pivots due to it being a lot less passive than Corviknight, Gliscor, and in some cases Tapu Fini.
What caused it to have a significant impact?
Zapdos' ability Static allows it to hinder many physical attackers, punishing opponents using pivot moves such as U-turn and Flip Turn as well as mult-hitting moves like Triple Axel and Surging Strikes. Being a Flying-type in a Ground heavy metagame is extremely useful and allows it to give teams a proper switch in to Landorus-Therian, Gliscor, Excadrill, Garchomp, and Mega Swampert. If Zapdos could only have one extremely beneficial partner, it would be Pelliper. With the ability to set rain, and pivot out with U-turn; Pelliper enables Zapdos extraordinarily well due to Weather Ball now allowing Zapdos to check Landorus-Therian and Heatran as well as 100% accurate STABs.
How do/did you deal with this Pokemon in NDOU?
Zapdos' struggles against basically any Ground-type due to them being immune to Electric-type moves, although they need to watch out for Hidden Power Ice and Hurricane. Some notable checks would be Garchomp, Landorus-Therian, and Heatran; Garchomp and Landorus-Therian despite being really weak to Hidden Power Ice, have access to Stone Edge which is severely threatening to Zapdos. Heatran is able to trap Zapdos with Magma Storm and use utility such as Taunt, Stealth Rock, and Toxic whilst recovering most of the chip damage with Leftovers, Zapdos can however pivot out with Volt Switch or U-turn as well as use Weather Ball to its advantage. A lot of the aforementioned Pokemon such as Kyurem, Gastrodon, Hippowdon, Clefable, Mega Diancie, Mega Tyranitar, and Ferrothorn are all great against Zapdos but I think this post is getting long enough as it is.