Demon/Doom 3

The Demon in Doom 3 (nicknamed the "Pinky") is very much different from its older versions. It has no visible eyes, its horns point straight forward instead of out from the side of its head, it walks on all fours in a canine style, and it has cybernetic rear legs. It roars frequently, messing with the player's perspective by shaking the screen.

It attacks in a similar fashion to its classic version by running up to the player and biting them, but due to the player's reduced movement speed, it is much harder to escape. Due to an increased attack speed and range, the attack is much harder to dodge. Additionally, the Doom 3 Demon is slightly larger than its classic counterpart, a fact which, combined with Doom 3's more cramped corridors, makes it tougher if not impossible to dodge around it.

Although the Demon's bite attack only does modest damage, being hit by an enraged Demon disrupts the player's aim, which is very confusing and makes a counterattack much more difficult. Combined with the fact that it attacks at a faster rate, it's an enemy that can attack with almost total impunity.

Like the Doom 3 version of the Imp, the Demon is tougher than its classic counterpart, but this is balanced out by the increased firepower of the game's weapons. It still takes only around two shotgun blasts at point-blank range to kill it.

Unlike their progenitors, which appeared in vast numbers, this Demon is a somewhat uncommon enemy. They are usually encountered by themselves, although the player may occasionally encounter 2 or 3 of them in a row. They generally do not attack alongside other enemies, but may appear after the player dispatches a wave of several lesser enemies (i.e. Z-Sec soldiers or Imps).

Tactical analysis
Demons are large and powerful, and normally encountered in small rooms or narrow corridors to minimize the player's movement abilities. Although in classic Doom games the chainsaw was extremely useful against the monster as the weapon shielded the player from the Demon's bites, in Doom 3 by contrast, the Demon is able to bite the player anyway; if the Demon gets within range to bite, the player will take damage and lose his aim. Because of this, killing the Demon using a ranged weapon before it has a chance to bite you is essential.

Using the pistol, your fists and the flashlight on an attacking Demon are not effective. The Chaingun and the Plasma Gun are the most effective weapon against Demons since the bullets and the cells can take them down in seconds, and the Demon stops to roar when its health is depleted to a certain point, giving the player more time to attack. The shotgun is decent enough in a pinch, and should take out a Demon with 2 to 4 shots depending on distance and accuracy. In Resurrection of Evil, the Super Shotgun easily kills them with one shot, as its large size will ensure most of the pellets hit.

Although Demons move slightly faster than the player normally walks, they can be outrun by sprinting.

Although seemingly large and limited to forward movement, demons can actually quickly hop sideways several feet when shot if there is enough room for them to do so. While this is rarely seen in the proper game due to the locations they appear in, this A.I. behaviour will appear in user created levels where is sufficient space. Thusly they can avoid weapons fire and make it difficult for the player to reorient themselves, leaving them open to attack.

Trivia

 * In the Doom movie, the character "Pinky" turns into a Demon towards the end of the film. It is clearly the same character, as the monster inherits his bionic wheelchair contraption. Why he did not simply mutate into a wheelchair-equipped Imp or Hell Knight is unknown; however, one explanation is that because there is less of him to work with than normal humans, the mutation was more concentrated on his "human" part.
 * In the Doom 3 E3 2002 video, in the bathroom, the Marine is hiding behind a corner as a Demon is eating the guts of a Fat Zombie on the floor. This is considered one of the most disturbing and memorable scenes in the demonstration. The Marine then emerges and destroys the Demon, taking little harm in the process. Surprisingly, the fat zombie then "awakens" and attacks, forcing the Marine to kill it as well.
 * The designers have explained the unique face of the Demon by describing how it is born: a newborn Demon's face is buried under thick folds of flesh (that do not, unlike other animals, undergo apoptosis), and the beast must then "consume" itself in order to breathe and eat.
 * It should be noted in some places in the Hell level of Doom 3, the organic half of this demon can be found hanging on meat hooks. Why they are there is unknown, but it has been speculated that the more intelligent hell-beasts use Demons as a food source. This is not likely, as Demons are very powerful and would disrupt tthe more intelligent hellspawns' attacks with its own attack.
 * Only three of these monsters appear in Doom 3: Resurrection of Evil, but as with the appearances in the original Doom 3, they are in very small corridors.
 * It is unknown how these creature piss or take a shit.