Hidden Pokémon: Mew And Celebi:

There are two Pokémon that appear in the Poké Balls but are extremely rare. Neither one attack but you can get special bonuses/trophies from seeing them. To get Mew to appear, you must first unlock all 11 secret characters. To get Celebi to appear, you must unlock every single thing in the game (except for trophies). After these needs are met, Mew and Celebi will both have a 1/251 chance of appearing in your Poké Balls.