Where do you catch unknown P on FireRed?

You can't catch P on FireRed! P is a Pokémon from the Pokémon Mystery Dungeon series, not the main series games.

It seems like you might be thinking of Mew, which is a mythical Pokémon that can be found in FireRed. You can get Mew by using the Mew glitch, which involves transferring Pokémon from the Generation 1 games (Red, Blue, Yellow) to FireRed. This glitch has been patched in some versions of FireRed, so it might not be available anymore.

Let me know if you have any more questions!