This will not affect most creatures in role-play.
So here, mythical creatures, full mythical creatures, cannot exist on Earth. It just isn't possible. And humans, and plain animals (not mutant animals), cannot exist in the mythical world.
However, if a mythical creature is born into Earth and does not know they are a mythical creature, so long as they are completely unaware of their powers they can live on Earth. But often they discover their powers, and are whisked away to the mythical world.