There's a secret ending in the game. If you talk to a man atop a climbable wall in stage 3, he will offer you a chance to partake in the Battle Arena. Doing so will warp you to an arena playing a remix of Vampire Killer from Castlevania, where you fight a three bosses:
A disco robot which explodes into fish pastries when defeated.
A zombie dinosaur robot pushing a machine-gun-toting carriage with a large slime monster inside.
A brain-robot which warps around the room.

After defeating the third boss, the player gets sucked into a portal and warped to the Mesozoic Era and becomes the king of monkeys after several years pass. The credits then roll.