John Romero's head on a stick
Romero's head is a thing that looks like John Romero's head on a stick. It appears in Doom II MAP30: Icon of Sin, where it must be shot with the rocket launcher to exit the level. It is located in a room that cannot be reached or looked into from the outside; the only way to see it is to use the noclip cheat code and go behind the Icon of Sin face.
The head has less health than a cacodemon (250 vs. 400), but each rocket will only do a little damage, so it takes at least three rockets to kill the head. When the head dies, there are explosions and the level ends.
The head alone is harmless, but usually with these heads come spawn shooters.
Romero's head has a sprite prefix of BBRN, and is also known as the Boss Brain. This is an insider joke: the "boss brain" of the level is the head of one of the bosses of id Software.