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Slowpoke is an easy one, but I find Slowpoke's ecological niche and very specific adaptation to be super neato, so I'ma mention them anyway.
I have no reason to doubt the Pokedex or what's been said about this creature, so I'm just gonna go ahead and say it eats damn near everything in the Water 2 Egg Group. Slowpoke 'hunts' by the water's edge, so it thus makes a living luring in fish that live in the shallows, reefs, and tide pools.
Slowpoke most likely depend heavily on Magikarp as their main source of food with fish like Goldeen, Wishiwashi, Barboach, and maybe Finneon getting ate up every once in a while. If I remember correctly Remoraid are supposed to be migratory or seasonal if they don't have a host, so there could be Slowpoke getting fat off of them.
Something I've noticed, but the territories of Carvanha and Slowpoke don't seem to cross over. Carvanha is a water/dark type, so I can only imagine that Carvanha is the one fish Pokemon that's capable of turning the tables on the dopey Slowpoke and swarming the poor, dumb, bastard before they have a chance to respond.