All boxers:
Press X, B, L, R when "Press Start" appears at the title screen.

All boxers:
Win the Bronze, Silver, and Gold belts.

Mike Tyson Challenge:
Defeat Mike Tyson and win the Gold belt to unlock the undisputed champ mode.

Fight as Mike Tyson:
Win the Gold Belt to unlock Mike Tyson in single player mode.

Fight as Iron Mike:
Win the  Mike Tyson  Challenge to  unlock Iron  Mike.