Elaborate OP:
Were they always real? If so, then we live in the Pokemon World, obviously.
Did animals just up and become Pokemon? Or did like twenty billion Pokemon just spring up somewhere? Are they as powerful and temperamental as they appeared in the show/games, or more ferocious? Is Arceus now actual God here, with omnipotent powers n shit?
If anything in that paragraph is true, Society is turned on its head. Because, fuck, dude, video games just came to life outta fucking NOWHERE. The philosophical and religious implications of this sort of shit are ridiculous. You'd have tons of people with different, equally likely explanations, and nobody with any real authority will be able to get a concrete grip on this.
Do Pokemon now follow real rues of biology? Do they DID in battles? Can bullets kill the ones that are soft-bodied, or do they just 'faint'? Will people even NEED to fight with Pokemon? Obviously ecosystems are going to get fucked bad. Economic collapse is almost inevitable when Pokemon begin to interfere with human resource cultivation; not to mention the number of Pokemon with are outright parasitic to human civilization and infrastructure.
It'd be a disaster. Unless you could befriend your own strong Pokemon, survival in the world to follow will be hard to guarantee. Wars will break out. Bombs will fall, and by that, I mean THE bombs, A Bombs. Nations and belief systems will crumble. Many people will be killed; most by the hands of Man.