Zombieman

The Trooper (sometimes known as Zombieman) is a former human who carries an assault rifle. Zombiemen are the first monsters encountered when playing Doom (unless you are playing on Ultra-Violence on E1M1 of Doom, in which there is a group of Former Sergeants to your left). It is arguable they are the weakest monster. When a Zombieman is killed, he drops a clip of bullets. However, clips dropped by Zombiemen contain fewer bullets (5) than standard clips (10).

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. This is because the Imp was referred to as the "Demon Trooper" in the Doom Bible.