Normally during the game you play the hero, Rex. However there is a way to play any of the other 16 characters in the game including Mondu, and fight against Rex. To use this trick, press the button sequences shown below:
Character Code
Bonapart UP, A, C
Brainiac RIGHT, RIGHT, C
Buff LEFT, C, B
Edwina LEFT, B+C, C
El Toro B+C, A, UP
Guano UP, RIGHT, A+B
McFire DOWN, C, RIGHT
Mondu A+B, DOWN, LEFT
Ramses RIGHT, LEFT, A
Robochic RIGHT, UP, DOWN
Sheba B+C, B, B
Skinny RIGHT, DOWN, RIGHT
Spidra A, DOWN, B
Stump A+C, RIGHT, C
Webra A+C, UP, RIGHT
Weezil DOWN, RIGHT, UP