Pokemonia is like any other life habited planet, exept for the planet's appearence, and the fact that Pokemon dwell on it as well as humans. In this world, that is where all of the pokemon stuff happens in the games in stuff. Every pokemon lives here.
What caused the formation of Pokemonia is still unknown, but what scientist do know is what causes all the pokemon to live here. It is that instead of Nitrogen in the air like Mazania or Ultima, there is pokemisum, which is a version of Oxygen, but for Pokemon as well.