There are "4" unlockable characters in this game, Eric Bischoff, Howard Phillips, Arlo, and Copper the Goat. Each of these characters can be unlocked by pressing buttons in a particular order on the title screen.
(Please note that this game does not feature the ability to save, so you will have to input these button combinations every time you start the game up if you want to play with them)
Up Up Down Up Up Down Up A
Up B Down B Left B Right B
Up Down Up Down A Right Left Select
Up Down Left Right Select Start B A