In the original series, the Demon has a hunched back with pink skin, clawed feet on its hind legs, a large head with sharp teeth pointing out, two large muscular arms, beady gold eyes, and two bull horns on the head. The Demon (also known as Pinky Demon, Shaved Gorilla, Bull Demon) is a well-known monster found in all incarnations of the series. In DOOM RPG, three variants of Imp exists. There are also the Nightmare Imps, which have a transparent purple/blue appearance to them, move faster, and fire purple fireballs, though they look essentially the same. In DOOM 64, there are the common Imps, which act like their counterparts, though their appearances are slightly different they have the same physical build and colour, but seem to be devoid of a mouth and several of the portruding spikes. They are referred to as 'spinies' and 'imps'. Their fireball attack is produced by spitting in their hand, where the mucus then turns into fire. In the DOOM novels, imps appear as they do in the older games - however, some can talk, as they are made 'leaders' of small groups. They attack at long distance by firing fireballs from their hands, and up close by scratching with sharp claws. They usually emit a hissing sound when alerted and have 60 HP.

In the original games, Imps have brown skin, red eyes, and spikes on their shoulders, elbows and knees. The Imp is the basic demonic enemy and common in all of the games because they appear in almost every level, where they appear with a relatively humanoid appearance.

Commando classes are named: Troop, Commando and Assassin. The Zombie classes are named: Zombie Private, Zombie Lieutenant and Zombie Captain. In the DOOM RPG two classes of zombies exist, each with three variants corresponding to their difficulties. The Former Commando does not appear in DOOM 64. The only real physical difference is that the Sergeant's pants are a slightly darker shade than the Former Humans (which makes them harder to discern within DOOM 64's darkness). In DOOM 64 there are no real changes in behavioural patterns amongst the zombies they simply walk and shoot. They also emit a 'sour lemon smell' when killed or nearby. However the former commandos depicted there (referred to as Clyde) are not zombiefied humans, rather they are either genetically engineered human copies or human traitors who have been genetically altered to look alike.

In the DOOM novels, zombies are found in various forms, ranging from former civilians to soldiers. (Appears in DOOM II, Final DOOM and the PlayStation and Saturn versions of DOOM) Also known as heavy weapons dude or chaingunner.

Former commando: 70 HP, wields a chaingun that he will drop when killed.

(Appear in DOOM, DOOM II, DOOM 64 and Final DOOM)

Former human sergeant: 30 HP, wields a shotgun that he will drop when killed.

(Appears in DOOM, DOOM II, DOOM 64 and Final DOOM)

Former human: 20 HP, wields a rifle, will drop an ammo clip when killed.

Three different forms of zombies are featured in the DOOM video games: Moving out of their line of sight, such as hiding behind a wall or using a lift, will break their lock and cease their firing, and they will be on the move to seek out their foe. Commandos, however, once within line of sight of their foe, will continuously fire their chaingun, only rotating to adjust to their target's movements. (The dropped ammunition, however, can be lost if a door or crushing ceiling comes down on it.) Owing to their usage of bullet weapons, zombies can be easily tricked into engaging in monster infighting with other creatures as well as their own kind.ДЇormer humans and sergeants will close the distance on their foe, and then circle, pausing every few steps to fire a shot. They are the only non-demonic enemies in the game as well as the only ones who drop ammunition (half the ammo of a fresh clip or a weapon pick-up) when killed. Zombies are undead soldiers who have been turned into brain-dead killing machines by the demons.

Doom monsters then and now