I think just about every mon we would want to counter already has a counter so this is hard. I don't think everything needs (or even can have) a counter as hard as Zapdos - Rhydon, but like... Pineguin and Gargoyle pretty significantly change a lot of dynamics. Anyway, in the hope of not having such rigid requirements as some other submissions, where they essentially MUST become a specific type and check a specific Pokemon, I'd like to pitch Aerodactyl, for 3 reasons:
1. It still does have an exploitable typing, so no team should get completely walked over by it
2. It already counters several Snorlax sets, but nowhere near all of them, and it still doesnt like para
3. With just mild upgrades, it could be an offensive check to various things (Jolteon, Zapdos) but would not be able to switch in, which I think is a better dynamic than just having a hard wall that prevents X Pokemon from doing anything, which is either hyperspecific (and therefore is a matchup fish) or is broken
I think we don't need to go the route of giving this QuakeSlide and massive stat buffs, it could be more of a Scyther-Scizor dynamic with evolution or just only get a few stat points on evolution and use it as an excuse to give it a move or two.