Monoculus is a villain from the Wingnut comics that comes to life in the episode Bat in the Belfry. He's said to be the supreme ruler of his species.
Monoculus was created when energy from April's Aeon Crystal's crystal attached itself to the image of the fictional supervillain Michelangelo's issue of "Wingnut an Screwloose". It was later destroyed by April O'Neil using her psychic powers.
Monoculus is also the name of the Demoman's haunted eyeball in the Steam game Team Fortress 2 after it was possessed by the wizard Merasmus' spellbook, the Bombinomicon.