"The one that seems the most plausible to me is that anything we recognize here is drawn from our memories. For everything you're not familiar with, you can find someone who is, right?"

Those naked chickens don't look like any sort of Pokemon, but there's people who recognize them at least partially, giving the trainers a hint as to what they're called. And the same thing goes for the various locations and landmarks of the world.

"So that's my guess -- that they're not really Pokemon at all, at least not in the sense that we know Pokemon." Who's ever heard of a Pokemon made of memories? (Although that sounds like the origin of some Ghost or Psychic Pokemon....)