Ready 2 Rumble Boxing: Round 2 [N64]

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.

Unlock Michael Jackson
To enable Michael Jackson, highlight Survival on the main menu, then press Left, Left, Right, Right, Left, Right, and the L and R triggers simultaneously.

Unlock SHAQ!
To enable SHAQ, highlight Survival on the main menu and then press Left, Left, Left, Left, Right, Right, Left, Left, Right, and then press the L and R triggers simultaneously.