We have no idea. People have tried to figure it out, but there does not seem to be a perfect answer. I did encounter a Pokémon of immense power that seemed like it could have been the source, but I have no way of truly knowing.

Pokémon exist like any animal or person does, but their biology is extremely different. They have powers that can only be described as magical, and their intelligence is nearly human level. This society is largely built around them, and they are a large part of work and entertainment.