Imp

The Imp is a human-sized humanoid demon and also the most common enemy in Doom games, thus being considered the "backbone" of all hellspawn in the entire franchise.

The Doom instruction manual says:
 * You thought an imp was a cute little dude in a red suit with a pitchfork. Where did these brown bastards come from? They heave balls o' fire down your throat and take several bullets to die. It's time to find a weapon better than that pistol if you're going to face more than one of these S.O.B.s.

Trivia

 * The Doom source code gives the object name MT_TROOP and the sprite root name TROO to the Imp, arguably because Imps were called "Demon Troops" in the Doom Bible, being the main and more common force of monsters of a directly demonic origin in the invasion. According those early plans, the Imps were meant to belong to a different species than the Demon Troops and were described as more traditionally impish in design; small flying demons, possibly like those eventually found in Heretic, which were apparently called gargoyles mainly to distinguish them from the existing Doom monster, since they look like commonplace imps.
 * Apart from zombies and Wolfenstein SS, the Imp is the only hellspawn that can be gibbed (e.g. by rockets, berserk fist attacks, barrel explosions and BFG blasts).
 * Imps are the only other monsters aside from zombies that emit two different alert and death sounds when spotting the player.
 * It is very difficult getting two Imps infighting together but, if one Imp manages to destroy a barrel (mostly via fireball attack) and the resulting explosion hurts any other Imps lurking nearby, they will turn against the one that blew up the barrel in the first place, and hurt themselves mutually through scratching (since projectile attacks are hardcoded to not deal damage in-between creatures of the same species).
 * The Imp gibbing death sprite contains a Zombieman arm.
 * In a ZDoom-based source port, if the player is killed by an Imp (monster is credited with delivering the blow that reduces player's health to 0%), one of two obituary messages will be displayed at the top of the screen depending on which attack the Imp used to kill the player: "[player name] was slashed by an Imp" or "[player name] was burned by an Imp" for the claw and fireball attacks respectively.
 * Imps seem to resemble Weevils from Torchwood, particulary their faces.
 * In Brutal Doom, Imps launch more realistic fireballs and also have a leaping claw attack which it uses at relatively medium-to-close distance, similarly to their newer version.
 * The Imp's death sounds are stock soundbites of a camel's grunt, at a lower pitch. This is also the case of some of the death sounds emitted by the former humans.
 * Imp's dying voice is simply a Bactrian camel mating call.

Doom RPG

 * Main article: Imp/Doom RPG

In Doom RPG, the Imp appears as a class of monster. There are three variations, identified by color:
 * Impling (green)
 * Imp (normal colors)
 * Imp Lord (red with green eyes)

Imps are most susceptible to attacks from shotguns.