Fat Boxer
At the Character Selection screen, press Right, Right, Up, Down, Right, R, R, L.
Skinny Boxer
At the Character Selection screen, press Right, Right, Up, Down, Right, R, L.
Zombie Boxer
At the Character Selection screen, press Left, Up, Right, Down, R, R, L.
Huge Gloves
At the Character Selection screen, press Left, Right, Up, Down, R, L.