By putting various DVDs, games and/or music CDs into the game, you can create secret monsters. Here are some of the monsters that can be created using this technique:

Baseball Boy: Field of Dreams DVD
ChaosDragon: Crouching Tiger, Hidden Dragon DVD
DuckFever: Cats and Dogs DVD
HipMew: Cowboy Bebop: The Movie DVD
Kasumi: DOA: Hardcore PS2 game
MahjongKoro: Britney's Dance Beat PS2 game
Metal Jell: T2 Extreme DVD
Miku: Fatal Frame PS2 game
Peacocken: Fischerspooner #1 CD
Pipo Ape: Ape Escape 2 PS2 game
Pirate Ape: Splinter Cell PS2 game
RygaRoller: Rygar: The Legendary Adventure PS2 game
Surfana: Justin Timberlake Justified CD
Try your own and see what you come up with.