Here's why:
* Unknown Pokémon don't exist in the game. The "Unknown" Pokémon are a fan-made concept, not an official part of Pokémon Pearl or any other official game.
* Pokémon Pearl has a specific Pokédex. The game features a set number of Pokémon you can encounter and catch, determined by the game's developers. There are no "Unknown" Pokémon on the game's list.
It's possible you might be thinking of a different concept. If you can tell me more about what you're looking for, I might be able to give you a more helpful answer.