Zombie is the typical name given to the creatures made by the mutagen created by Dr. Curien. They are the main enemies in the game, being created (or in certain cases reborn) from the mystery compound from Overkill.
The mutagen causes necrosis, bleeding and aggression. Although it is unknown whether the creatures are the dead reanimate and feed on human flesh they certainly display several common characteristics of the traditional zombie. They have varying skin colours in each game, the standard ones walk with a lumbering slowness although other breeds are fast. Unlike zombies featured in other games, these zombies have shown minor intelligence, such as being able to utilize terrain and weapons. Another departure from traditional zombies is that these zombies actively pursue their targets with the intent of killing them, as many of them utilize weapons and other blunt objects to attack.
These particular zombies are shown to be different from traditional undead in other ways - while Zombies are usually stopped dead with a shot to the head (or the removal of it) most if not all of the Zombies in HOTD can survive, and actively pursue their targets, after having gaping holes blasted through their heads, or in fact the entire head being blown off, although this only usually occurs in the tougher ghouls. They are also suprisingly more intelligent for the most part, having the ability to use tools and weapons, as well as being able to negotiate obstacles and cover with ease. Some (like the axeman zombie) appear to have the drive to defend themselves, protecting their faces with their axes. They are also only partially shambolic - most run or pace towards their targets, while others shamble for a while until shot, after which they will hastily pursue the attacker.
It is not quite clear how the undead are created, although rows of vats appearing in both the DBR buildings containing the zombies in the second and third games suggest they are transformed from regular humans into the undead. However, it is likely that these vats are merely for display/testing purposes, or for the fabrication of specialised zombies (since the only kinds that burst from the vats in the third game are the Corpse and Leaper variants). Since bitten humans (both players and saveable civilians) do not turn into undead as a result, it is safe to assume that the virus, if it even is a virus, is conveyed by other means, such as through the water supply (which, considering the heavy mutation of the Mudmen zombies that dwell in the water, is a likely explanation) or a temporary airborne agent (which may have infected the undead in the DBR facility in the third game some time prior to the players arrival, and had dissappated by then - which also explains the lack of saveable civilians.).
Alternatively, it is possible that the zombies in the game are actually degenerated, lab-grown human beings, left with a debased hunger for flesh while retaining enough intelligence to, say, wield a weapon or drive a car. With Curiens limited cloning abilities leaving them (and in perhaps in Goldmans case, deliberately) genetically unstable, with flesh and sinew breaking down while they still walk and brain tissue rotting to the core, they have turned into cannibalistic, grunting and shambolic beasts hellbent on killing anything that isnt them. Some are more affected than others, such as the Ebitan, Kageo and David series' - the simplest, slowest and most common undead, whilst more specialised and developed types (such as the obese yet agile Bob, the chainsaw-wielding Max and the vehicle-using Ricky) are kept in a higher state of mind, but not high enough that they can overcome their maddenned bloodlust. This is backed up by the undeads apparent inability to transfer any sort of infection, and the vats of different undead in the DBR buildings in the second and third games.
Depending on the settings of the game, Zombies may have green blood. This is likely a censor, and can usually be turned off after the game is completed, making the blood red.
- Regular - Slow, weak and often coming in various shapes, the regular zombies are usually the most common enemy in the game. They are always unarmed, and are easily put down with one headshot in most of the games. They attack by biting, clawing, kicking and shoulder-barging the player.
- Muscle/Soldier - Bigger and tougher Regulars, these zombies are noticably taller and more muscular than the others. They often appear shirtless, or in semi-military attire (In the second game some wear military berets, in the third some have camoflauge pants and a flat-top haircut.). They take significantly more firepower to take down, and usually attack by punching or bludgeoning the player.
- Fatmen - Like the name suggests, these tubs of lard are encountered during every game. Like the muscle zombies they take a lot of firepower to bring down, and are often seen fighting alongside them. They often toss barrels at the player from a distance. Suprisingly, and humourously, some move and attack like undead ninjas - they sometimes sidestep towards the player like a boxer before delivering a punishing spinkick.
- Axeman - Appearing in every game, the axeman zombie comes armed with a pair of axes/hatchets. They use these weapons to shield their vulnerable heads, prompting the player to aim for the arms or the chest to expose the cranium. At a distance, they will lob both their axes in rapid succession at the player, which can be shot down. A unique type appears in the third game - it only has one large axe, and is unable to throw it. It is noted that, throughout all the games, most of this type of zombie encountered wear the same checkered shirt.
- Kageo - Appearing in all games (except Overkill), these lean, rotting monsters often appear in dank, dark areas such as sewers and abandoned laboratories. They are among the weakest in the game, taking only a few shots to dispatch. They have a consistent appearance throughout the games, a hunched-over swagger with medium-length balding hair and rotting skin. Some of these zombies can survive without their legs, crawling around for a suprise attack. A unique type is found in the second game, an upright variant armed with claws and an iron mask, which protects it from headshots.
- Ebitan - Appearing in all of the games (except Overkill), Ebitans prefer to stay in the water. Their attacks often involve leaping out of fountains and canals to attack the player. They take a lot of damage, and move as fast as Regular zombies even on land. In the third game, there appears to be a smaller variant, which shuffles toward the player before leaping at him/her.
- Knife Throwers/Leapers - Arguably the most annoying undead enemies, the Knife Throwers and Leapers are sometimes fused into one (irritating) type of foe, although in the second and third games they appear to be seperate. The Knife Thrower will usually move around at a rapid pace dodging the players attacks, advancing until it can take a slice at the player, all the while lobbing an infinite supply of sharp things. The Leaper, on the other hand, usually travels with a buddy, and likes to move even quicker, climbing along walls and hanging from the ceiling to reach the player quicker.
- Chainsaw Zombie - Probably the toughest enemy in all of the games, the Chainsaw Zombie runs at the player relentlessly to cleave them in two with his motorised death-machine. They have possibly the highest health of the zombies, making it all the harder to bring one down before he digs into you.
- Murrer - An ugly, slimy worm that crawls around in packs, the Murrer will slither around for a bit before jumping to bite the player. They are mostly just a nuisance unless theres a ton of them, which there usually is.
- Animal Zombies - Bats, owls, frogs, vultures, giant cockroaches, massive green Harvestmen spiders - you name it, Curien and Goldman are throwing them at you in force. Although it is unknown wether they attack you due to an infection by Curiens "virus" or through some connection to the Emperors "rule over nature", they can become dangerous if not disposed of quickly.
- Sledgehammer - Unique to the first game, the sledgehammer zombie will swing around a gigantic mallet attatched to a chain before hurling at the player, then dragging it back to repeat the process. They almost always attack from a distance.
- Broadsword - Unique to the second game, this particular zombie wields a massive broadsword, which it often uses to protect its face while it advances. It becomes vulnerable when (slowly) swinging the massive weapon around.
- Lizardmen - Unique to the second game, these peculiar lizardlike undead hang from pipes and whip the player with their "tails". They possess unusual resilience for their size, being able to get back up and continue attacking after being knocked down multiple times. Regardless of the settings, their blood is always a dark, muddy green.
- Parasite - Unique to the second game, the parasite zombie is infected with a giant worm in its chest (a reference to the Chestburster from the Aliens series). If shot anywhere other than the head, it will become temporarily invulnerable before ejecting the worm from its chest and dying. Its toughness is the same as a regular zombie - with good aim, it will prove to be no more dangerous than one.
- Commando - Unique to the third game, the Commando is a cross between a leaper and a muscle zombie. Lacking the multi-surface movement abilities of the former (but inheriting the speed) and the strength and getup of the latter, the Commando attacks with impossible speed, phasing from side to side and rolling around before slashing at the player. They are difficult to hit, but one good shot will stop them long enough for you to fill them with shrapnel. Their uniforms suggest they were the former members of Rogans tactical assault team, who were killed and zombified while trying to reach the Wheel of Fate. A unique type is found in the last level of HOTD III, taking an absurd amount of damage and standing back up multiple times before succumbing. It is assumed that this is the corpse of the tactical team member killed by the Undead at the very start of the game, or a zombified Casey, Rogans partner throughout the prologue level.
- Crawler - Unique again to the third game, the crawler hangs on to the ceiling and attacks by descending from above and clawing at the player. They are usually encountered alone, and at rare occassions in a pair or a trio.
- Flyer - Appearing similar to Manbat, this zombie glides about on a pair of rotting batwings before descending on the player. It becomes difficult to shoot the zombie as it is as quick roving around in the air as it is when it dives to attack you.
- Cyber-Zombies - Exclusive to the final levels of the second and fourth games, the Cyber-Zombies are upgraded - or scratch-built - versions of other kinds of undead. They appear to be constructed out of metal, and have a visible brain encased in what appears to be perspex, which also encases part of one leg and one arm. The regular version seems to be able to walk through walls, although this may be teleporting from a further distance, or remaining invisible until moving to attack the player. There is also a knife thrower/leaper hybrid variant, which seems to shift in and out of planes of existence, becoming bulletproof when "phasing" in. They toss electrified boomerangs at the player, and appear to defy gravity by walking on the walls and ceilings of the DBR buildings corridors. There is also a chainsaw variant, with a Cyclops-esque visor and a pair of electrified longknives in place of dual chainsaws.