A big floating monster that appears in Doom, Doom II and Doom 3. It's red, with horns and a huge mouth. It's also one-eyed. It moves slowly and its only attack is throwing a white fireball at you. They aren't very tough, but if you don't move they'll toast ya.
In Doom 3 they look different, smaller, and their attack is more powerful.
I hit that switch and two cacodemons appeared.