In the SNES version of the game, when the credits scroll pauses on "SOUND CREATOR ATOMIC HANADA", the game checks if X/A/L/R are all being held. If they are, the game shows an alternate ending sequence featuring Joe and Mac being chased away by four ugly cavewomen.
The final boss is actually changed depending on which console you have. In the Arcade, Genesis, Amiga, and NES versions, the final boss is a caveman-like monster with reptilian features. In the Gameboy version, it is a beating heart. In the SNES version, it is a devil-like monster.