- "Hellspawn" redirects here. For "Team Hellspawn", see Doom voxel project.
A monster is any artificially intelligent thing that is not spawned if the -nomonsters parameter is used. In single player and cooperative games, much of a user's playing time is spent in combat with enemy monsters. Monsters are either placed in a level during its design, created by another monster (particularly, the pain elemental spawns lost souls), or spawned by the final boss.
Any monster placed in a level or created by the final boss is initially dormant and becomes active only when it sees a player, when it sustains a damaging attack, or when it hears a player attack, even if this is with the (otherwise silent) fist. When activated, a monster will advance on its target and attack, sensing the location of thing they are after even if it is out of sight, and advancing toward that location even if it is on the other side of the level. If a monster hits an obstruction during its hunt for its target, it will attempt to walk around it. Monsters that have been alerted normally remain active indefinitely, even after respawning or being resurrected by an arch-vile. However, there is a bug which returns any active monsters to a dormant state if the user re-loads a saved game. In general, monsters of different types can accidentally hurt each other and will often turn against each other in such cases, engaging in monster infighting. If a monster defeats an aggressor without ever seeing or hearing a player, it will return to a dormant state.
Monsters can use some types of teleporters, although they do so accidentally, as their artificial intelligence is very limited. Monsters can similarly activate lifts and open certain doors, although some monsters may not fit through doors and openings that are just large enough for the player. Monsters can be hurt and killed by crushing ceilings, but are unaffected by all forms of damaging floors.
 Doom games
The monsters of Doom can be classified into two groups: the former humans possessed by demonic forces, and the demonic monsters (or hellspawn) originating in hell or arriving from there.
- Former Human: Once a marine or guard like the protagonist, now a green-haired, hell-possessed soldier carrying a rifle. The weakest of all enemies and therefore, the easiest to kill.
- Former Human Sergeant: A bald, more durable zombie in black armor with a shotgun. A bit stronger than the former human and moderately dangerous at close-range, but still very easy to gun down.
- Imp: A brown, human-sized humanoid demon that throws round fireballs and claws opponents at close range.
- Demon: About the size of a gorilla, this muscular pink brute runs quickly on its two hind legs to bite opponents.
- Spectre: Identical to the demon, except that it is partially invisible.
- Lost soul: A floating, flaming skull that can rush quickly to gore at an opponent.
- Cacodemon: A large flying head-like monster with red scales, horns, and one eye. It moves relatively slowly, but has strong jaws and spits out "lightning balls" from a distance.
- Baron of Hell: Two of these act as the boss in Knee-Deep in the Dead, resembling large pink satyrs. It can hurl streaming balls of green flame or energy and employs a powerful clawing attack at close range.
- Cyberdemon: The massive boss in The Shores of Hell, a monstrous humanoid cyborg as tall as two men that can absorb more damage than any other monster and moves at a relatively rapid rate for its size, pausing only to fire rockets from its mechanic arm at anything foolish enough to oppose it.
- Spiderdemon: The cybernetic boss in Inferno and Thy Flesh Consumed. It appears as an enormous brain with eyes, a mouth, and comparatively small arms, atop a four-legged metal chassis with an incorporated triple-strength chaingun.
 Doom II
- Heavy weapon dude: A tough, corpulent zombie with a darker skin, red armor and carries a chaingun.
- Hell knight: Identical to the Baron of Hell but tanned in color, emits higher-pitched sounds and endures only half as much damage.
- Revenant: A tall, skeletal demon wearing armor that can launch ballistic or guided missiles, and punch opponents when at close quarters. These are the results of recycled demon combatants.
- Mancubus: A grotesquely bloated humanoid with flamethrowers instead of forearms. It moves slowly, but takes a fair amount of ordnance to bring down.
- Arachnotron: Strongly resembles the spiderdemon, but is much smaller, and shoots greenish plasma instead of super-chaingun rounds.
- Pain elemental: A large flying head resembling a brown cacodemon with small arms. It attacks by spitting lost souls at its target, and even creates a few more as it dies.
- Arch-vile: A yellowish, emaciated humanoid demon who is quite tough for its size. It can summon bright yellow flames to engulf and blast targets, and knows how to bring dead monsters back to life. It is the fastest monster in Doom II (almost one and a half times as fast as the cyberdemon).
- Wolfenstein SS: Populates the two Wolfenstein-themed secret levels and attacks with a medium-strength machine gun.
- Commander Keen: A hanged effigy of the cosmonaut from id Software's pioneering series of games only appearing in MAP32: Grosse. It neither moves nor attacks, although killing all four Keen figures is necessary to reveal the exit switch and finish the level.
- The final boss is not technically a monster, but does still represent one of the protagonist's opponents.
 Sony PlayStation
The ports of Doom and Final Doom to the Sony PlayStation lack the arch-vile, but include an additional monster over the PC versions:
 Doom 64
Doom 64 includes newly drawn versions of the Doom and Doom II monsters. The revenant, heavy weapon dude, arch-vile, spiderdemon, Wolfenstein SS, Commander Keen and final boss are not present, but instead it features some new monsters:
 Newer Doom franchise games
The newer games of the Doom franchise generally include a set of monsters from the PC Doom games or inspired on them and possibly a number of new creations. Various ports include the same sprites as their predecessors, although some change their appearances either to achieve a new style (such as in Doom 64) and the newer games (such as Doom 3) provide polygonal versions that suit their more advanced technology and rendering capabilities.
 Doom RPG
Doom RPG features almost all of the monsters from Doom II, with the exception of the shotgun guy, spectre, arachnotron, Wolfenstein SS, Commander Keen and spiderdemon (as well as the final boss). It includes the following unique monsters:
Non-boss enemies in Doom RPG are organized into classes. For example, there are three types of imps, while the hell knight and baron of hell both belong to the "baron" class. Monsters in the same class use the same sprites, but are somewhat differently colored and vary in difficulty.
 Doom II RPG
The Doom RPG prequel Doom II RPG has almost the same enemy entity set as Doom RPG, but adds a few of its own, even adding a few boss entities. The game adds the following unique NPCs:
- Lesser Sawcubus
- Greater Sawcubus
In addition to the new classes of enemies the game has two new boss class entities; the Pinkynator and VIOS.
 Doom 3
Doom 3 includes its own versions of many monsters from the original games. Some of these, such as the demon and the hell knight, are considerably different from their predecessors.
- Hell knight
- Lost soul
Additionally, the following new monsters are encountered:
Some concepts of monsters, one of which resembles the spider-like monsters from Doom and Doom II, were shown by id Software during development of the game but did not make it to the finished product.
 Doom 3: Resurrection of Evil
All the monsters in Doom 3 appear in Resurrection of Evil, except the sabaoth, vagary, cyberdemon, guardian, seeker and lost soul. Additionally, some new monsters are included:
- Bio-suit zombie
- Forgotten one (which resembles the classic lost soul more than does its namesake)
- Hell hunter
 Other games based on the Doom engine
Games powered by a modification of the Doom engine include a particular set of monsters suiting their setting, style and game behavior. These monsters share various basic characteristics of the Doom game monsters but may show some differences, such as being capable of multiple types of ranged attacks or being subject to the additional possibilities of game scripting (namely ACS in Hexen).
- Disciple of D'Sparil: Apprentices and followers of D'Sparil. They have mastered the ability to fly and cast a wave of bone-crushing missiles at their target
- Fire Gargoyle; A stronger, but visually identical variant of a gargoyle that throws small fireballs instead of attacking in melee.
- Gargoyle: Small red imps that have the ability to dash at a target and rip at them with their claws
- Golem: A humanoid creature made of rock that can smack you hard.
- Golem ghost; A ghost version of a golem.
- Iron lich: Giant metal skulls able to attack with a variety of elemental-themed attacks.
- Maulotaur: A huge bull-like creature that wields a huge hammer.
- Nitrogolem: A golem that has mastered the ability to conjure up flaming skulls that track their target.
- Nitrogolem ghost: A ghost version of a nitrogolem.
- Ophidian: Human-sized snakes wielding staves that slither around the Dome of D'Sparil
- Sabreclaw: An automaton of unknown source with blades for appendages.
- Undead warrior: The dead brought back to life and given armor and magic axes.
- Undead warrior ghost: A ghost version of the undead warrior
- Weredragon; A hulking beast that belches fireballs.
- D'Sparil: The first Serpent Rider and chief antagonist of Heretic.
- Afrit: Black bats on fire that dart around and launch streams of fireballs.
- Brown chaos serpent: A wild and hostile creature native to the world of Hexen. It belches poisonous gas.
- Centaur: A four legged creature in armor that carries a sword and shield, which it can use to protect itself from harm.
- Dark bishop: Powerful leaders of the former Church, warped into mindless servants of Korax.
- Death wyvern: A large undead flying creature with a thirst for the blood of the living.
- Ettin: The mutated remains of the Legionnaires.
- Green chaos serpent: A wild and hostile creature native to the world of Hexen. It spits powerful fireballs.
- Heresiarch: Blasphemous beings that control forces capable of sending mere mortals screaming in terror.
- Reiver: Undead creatures that fly through the air and pelt you with fireballs
- Slaughtaur: A centaur that has mastered the ability to launch energy bolts from its shield.
- Stalker: Aquatic reptiles with huge talons designed to rend their prey into bite-sized morsels.
- Stalker boss: Same as the above but also has the ability to throw slime projectiles.
- Wendigo: Creatures made of ice that launch ice balls from their hands.
- Menelkir: The former leader of the Mages guild, given the gift of unlife by Korax.
- Traductus: The former leader of the Church, given the gift of unlife by Korax.
- Zedek: The former leader of the Legion, given the gift of unlife by Korax.
- Korax: The second Serpent Rider and chief antagonist of Hexen.
- Maulotaur: Players can summon these powerful creatures to defend them.
- Acolyte: The Order's primary guards and soldiers. Armor colors indicate rank.
- Ceiling turret: A armored turret, on the ceiling.
- Crusader: Totting a long range missile launcher and a flamethrower, crusaders are trouble from the moment they show up.
- Inquisitor: A prototype of the Order's latest mechanical menace.
- Reaver: Fast-moving humanoid robots.
- Sentinel: They fly. They shoot and they seem to be everywhere.
- Shadow acolyte: Acolytes with cloaking technology.
- Stalkers: The best reason yet for arachnophobia; they can walk along and jump to and from the ceiling.
- Templar: A mauler-carrying demonstration of the Order's mastery of technology.
- Majong 7
- Roaming mine
- Thorn thing
- Unnamed invisible enemy