Hold L1 and press Up, Right, Left, Down, Square, Square, R1, R1, R1 at the character select screen.
First enable the Mother Mosquito code, then hold L2 and press Up, Right, Left, Down, Square, Square, R2, R2, R2 at the character select screen.
Two player mini-game:
Hold Start + Select on controller two after turning on the system. During the game, press Square to move forward, X to jump, and Triangle to do a super attack.
Thanks to Revolution reader Dustin Whaley!