Mancubus/Doom 3

The Mancubus (plural: Mancubi) is one of the few Doom 3 monsters that remain faithful to their original designs, taking an overall appearance of a very large, obese, bald humanoid with large elephant feet, armed with fireball launchers for each arm, that emits a loud low-pitched grunt and roar when they are present. The Mancubus stands about two heads taller than a normal human, and its bulk is about 2-3 times as wide.

The main differences appearance-wise. between the Doom 3 Mancubus and the Doom II are the skin color and facial features. While the Doom II version had a brown leather colored skin and a humanoid face, the Doom 3 version has a pale green skin tone, and dark green eyes that seem narrowed to almost slits. It also has a thick trunk-like appendage that connects its mouth to its chest, which according to the game designers is a breathing tube featuring several small holes along its sides (which are visible if viewed closely), with tusks on both sides of its face, making them resemble elephants. Due to a graphical and model design glitch, these tusks were reduced to feeding tools, and are stated to be used like the mouth of an octopus.

Mancubi are quite rare, and only appear during two levels in Doom 3. The Mancubus makes its first appearance in Recycling Sector 2. The Marine enters the area when he begins to feel tremors and a low pitch grunt coming from the monster. Suddenly, the creature crawls out from a waste duct, literally headbutting its way out, then stands up to fight the player. Several are also encountered throughout Hell. They also appear in Resurrection of Evil and The Lost Mission, although they are mostly confined to Hell.

Combat characteristics
The Doom 3 Mancubus attacks very differently from its classic iteration; while the original version is capable of firing its two flamethrowers simultaneously, the Doom 3 iteration can only fire from one cannon at a time, before firing the second shot from the other cannon. It will continue alternating by firing a steady barrage from each cannons, until the player is out of its line of sight.

Unlike the classic iteration, this version has its own melee attack: the monster will lift up both of its cannons and will smash its target with all its strength, but not after going through its long "recharge" time.

Because of their large size, the Mancubus can also make the ground tremble from their loud banging footsteps.

Tactical analysis
Contrary to the classic Mancubus, which often posed as a middle-tier monster (similar to the classic Hell Knight), Doom 3's version is a real powerhouse, as it has the highest health out of any non-boss enemies in the game. Their powerful ranged attacks can quickly incinerate a healthy player within a matter of seconds, and their melee attacks is most damaging out of all enemy (second only to the Cyberdemon's stomp). Players are fully justified in using the strongest weaponry in their arsenal to bring one down.

The weapons that are effective against the Mancubus are the Chain Gun, Rocket Launcher, Plasma Gun, and the BFG. Grenades also work well due to the lumbering speed of these behemoths. Headshots are effective and quite easy to score on a Mancubus, since this creature is a slow-moving and very large target. A Mancubus can be killed with about 4 Rocket Launcher Headshots. If the Soul Cube or Artifact is available, by all means, use it.

Close combat is not recommended against the Mancubus, thanks to the monster's high HP and extremely powerful attacks. However, under extreme situations should the player be forced to fight the Mancubus up close, the Shotgun can work well against these behemoths. You can run and circle around it, then use the Shotgun to hit its back. It will turn to your direction and attack you, however, you can run the opposite way it turns to keep you facing its back (take example: if it turns right, run left) and keep firing the shotgun to finish it off. Areas with columns improve the effectiveness of this tactic.

A Mancubus can be killed with just two of its own fireballs thrown back at it with the Ionized Plasma Levitator.

Trivia

 * If the Mancubus model is examined more closely (e.g. using the noclip cheat or a model viewer), it's possible to see hooks with suckers where the trunk begins, suggesting the trunks are actually feeding tools. Also, when the Mancubus shakes its head, they flap around. This could indicate that the design of the Mancubus was inspired by Cthulhu, like in Quake. The tentacles can also be seen in the beast's introduction FMV.
 * Some of the original Mancubus grunts from Doom II were used in the Doom 3 version.
 * These monsters are usually accompanied by Cherubs in the original and in one level of Resurrection of Evil, Erebus Research. This has lead to speculation they are the offspring of Mancubi. This is most likely coincidental, since they look and behave differently.
 * There are only 10 Mancubi in Doom 3 (not counting expansions): 3 in Recycling Sector (in "their" room) and 7 in Hell (one before the Berserker room, 4 on the hammering pillar room, one on the fiery large room after the Cherubs corridor, one behind the door in this room, and the last one in the corridor next the end of the level).
 * The Mancubi that appear during the sewage tunnels segment in the level Erebus Research from Resurrection of Evil bear a distinctive dark green skin tone (presumably due to being coated in toxic waste), and only appear in that particular section of the game. This would make them one of the only 3 demon enemies in either Doom 3 or its expansion pack to have area/level-specific alternate skins, with the other two being the rune-scarred Hell Knights and Imps that appear exclusively in the Hell levels. (This does not apply to the zombie variants, as they are different models altogether).
 * Whereas the Mancubus in Doom 2, Doom 64 and Doom Eternal features six small saggy breasts on its chest, the one in Doom 3 only features two.