Zombieman

The Trooper (sometimes known as Zombieman) is a former human who carries a pistol. Troopers drop a clip of ammo when they are killed, which can be picked up by players. Troopers are the first monsters encountered when playing Doom. It is arguable they are the weakest monster.

Internally the Doom source code gives the Trooper the type name MT_POSSESSED. The sprites for the trooper have the prefix POSS.

Interestingly, while this enemy is commonly referred to as the "trooper", internally the name MT_TROOP (and the sprite prefix TROO) is given to the Imp.