Former human

Former humans, as a conceptual family of enemy types, are humans that have either been slain and reanimated, or "zombified", or have been outright possessed by the evil forces of Hell, and taken as a group they are Doom's most common enemies, especially in early levels.

Description
They were once Earth's soldiers or other formed of armed military personnel, but have been somehow turned undead/or have been possessed and now serve the demons they once fought. Inside the Doom source code and in the Doom Bible they are called possessed humans, and their sprite prefix always contains an abbreviation for "possessed". Because of their immediately human origin, they attack with hand-held firearms instead of innate abilities or cybernetic implants as other monsters do. This provides them the advantage of being able to hit the player with better accuracy and timing from the hitscan of their firearms. Conversely, their inferior physique such as low speed and durability, makes former humans easier to eliminate than most of their fully demonic counterparts.

Former humans drop weapons or ammunition when killed; this sets them apart from the purely hellish monsters. A dropped item remains even if the zombie respawns or is resurrected by an Arch-vile; if killed again, the zombie will drop yet another item. Being humans that somehow became possessed by demonic forces, they are not generated by the final bosses of Doom II or Final Doom's episodes.

Classic Doom
The military forces on the moons of Phobos and Deimos, and on Earth in Doom II and its sequels, were possessed en masse when the invasions of those areas began. Zombification was known of before the invasions, however, and was noted during the earliest teleportation experiments. In the game's instruction manual, test subjects were said to become violent, babble vulgarities, and eventually suffer from "random full-body explosion." The latter mechanic does not actually occur in-game.
 * Zombieman
 * Shotgun Guy
 * Heavy Weapon Dude

And, relatedly:


 * Wolfenstein SS

Doom 64 also has its own versions of the Zombieman and Shotgun Guy, which are functionally the same as those from Classic Doom but look slightly different.

Doom 3
In Doom 3, possessed humans belong to one of three main classes: ordinary unarmed basic zombies, firearm-bearing Z-Secs, and the elite commandos. The first two are created through direct possession by evil spirits, which appear as translucent flying skulls at the start of Hell's invasion. Commandos on the other hand were created directly by Dr. Malcolm Betruger, through use of demonic magic. As such they display a higher level of intelligence and brutality, and share more in common with demons than with the lesser possessed humans.

Simply visiting Hell could result in a gradual form of zombification similar to that described in Doom's instruction manual. Instances of this increased as the Delta Complex's experimentation with teleportation increased in frequency, and the UAC's doctors were unable to explain the process.

Doom (2016)
See also: The Possessed and Revenant

In the 2016 reboot, the Possessed are a class of monsters who were created from the bodies of UAC employees subjected to Lazarus and Hell waves. In the hierarchy of Hell, the possessed are the lowest level and treated as slaves, forced to perform labor such as constructing temples or tending to gore nests. Possessed who are taken to Hell eventually further transform into the Unwilling.