Champion costumes:
     Beat the game in championship mode to unlock the champion costumes.

Fat boxer:
     Press  Right, Right, Up, Down, Right, R1, R1, R2 at the character
selection screen.

Skinny boxer:
     Press  Right,  Right,  Up,  Down,  Right, R1, R2 at the character
selection screen.

Zombie boxer:
     Press  Left,  Up,  Right,  Down,  R1,  R1,  R2  at  the character
selection screen.

Huge gloves:
     Press Left, Right, Up, Down, R1, R2 at the character selection screen.