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

Fat boxer:
     Press  Right,  Right,  Up,  Down, Right, R, R, L at the character
selection screen.

Skinny boxer:
     Press  Right,  Right,  Up,  Down,  Right,  R,  L at the character
selection screen.

Zombie boxer:
     Press  Left,  Up, Right, Down, R, R, L at the character selection
screen.

Huge gloves:
     Press Left, Right, Up, Down, R, L at the character selection screen