Abhumans are human subspecies - descendants of humans who have physically evolved after long periods of isolation on worlds with various extreme environmental conditions.
The Imperium recognizes 73 stable abhuman strains, of which 46 types are now listed as extinct, with a further 12 suggesting they too have died out or been assimilated into the greater population. The status of the remaining 15 strains is an issue of constant debate within the Administratum's Tithes Chamber Notaries, sub. Planetary Census (Abhumans), the administrative body that deals with Abhuman affairs.
In an Imperium where genetic mutation and spiritual corruption are often interrelated, abhumans are a focus of much controversy. In more liberal times under the Emperor, even markedly divergent abhumans such as the Beastmen could serve in the Imperial Guard. Since the Emperor's stasis however, only the lesser deviants, such as Ogryns and Ratlings, are considered Imperial citizens. However, they are still distrusted by the more puritan members of the Inquisition.
Imperial Guard armies may make use of Abhumans. These Beastmen (or Homo sapiens variatus), Mutant Slave Levies (ordinary mutant slaves rounded up for cannon fodder), Feral Ogryns (even nastier and more primitive than regular Ogryns), Nightsiders (originating from worlds of perpetual or near-perpetual darkness, little to no ability to see but perfect warriors for Night Fighting missions), Afriel Strain Soldiers (genetically engineered from DNA taken from great heroes of the Imperium, very unlucky and unpopular), Subs (relatively genetically stable but still hideously deformed mutant sub-breeds) and Gland Warriors (Imperial Guardsmen implanted with special organs and glands that secrete combat-useful chemicals, such as stimms and pain-killers).
Afriel Strain (Homo Sapiens Maledictis) soldiers were created during an experimental process, of which little is known about. It was an attempt to capitalise on the characteristics of the heroes of the Imperium's genetic material, using all the best traits which made them great - Faster, stronger, and more intelligent than baseline humans . Afriel Strain warriors are known for having very bad luck. Afriels are normally albino, with pale alabaster skin, pale hair and colourless eyes.
Beastmen (Homo Sapiens Variatus) are bestial abhumans combining the forms of animal and human. They were formerly recruited for the Imperial Guard, where they were valued mainly for their bloodlust; they fought for the Emperor in order to atone for their sin of being born mutants. They were led by Beastmen called packmasters. Beastmen are no longer seen in Imperial service; no doubt elements of the Inquisition convinced the Administratum that they couldn't be so radically altered from normal humans without being tainted by Chaos. Today, Beastmen are the subject of severe Imperial persecution and are hunted down by the Adeptus Arbites.
Members of the Necromundan House Goliath, these freakishly large vat-grown warriors can pass a even a Space Marine in size. They also suffer from a variety of defects, most notably mental imbalances, slow movement and extremely short lifespans, thus, they are considered by many to be a classifiable strain of Abhuman.
Nightsiders, (Homo Sapiens Tenebris) descend from the populations of many worlds in the Imperium that live their lives in near, or sometimes complete darkness.
Some planets are inhabitable only on their night side, because the opposite surface is exposed to the full force of the system's star. Other populations exist underground, because the surface of the world is entirely uninhabitable. Yet others live on nomad worlds, which wonder the galaxy independent of any star. 
Throughout the millennia, technology may have regressed due to ignorance, isolation, or natural disaster, and the population may have changed by design or by some accident of nature. Such populations are considered mutants by many, for they may sport bulbous eyes adapted to the dimmest conditions, or they may in fact have only vestigial eyes and enhanced senses - such as Echolocation - that compensate for near or total blindness.
Ogryns (Homo Sapiens Gigantus) evolved on cold, high-gravity prison planets. They are huge and stupid but extremely loyal once introduced to the Imperial Cult. It's even said that those who fought as rebels in the Horus Heresy only did so because their superior officers lied to them about which side they were on. Ogryns are used as shock troops, wielding heavy automatic shotguns called Ripper Guns, which can be used as clubs. They tend to be claustrophobic and thus are difficult to transport in Chimera APCs. They are led by Bone'eads, who have undergone Biochemical Ogryn Neural Enhancement so they're not quite so stupid.
The issue of Ogryn classification is one of the most contentious within the Administratum's Abhuman department. This complex strain is currently listed as seven distinct types (Alpha, Theta, Type IV, Type VIIa, H.S. gigantus gigantus, H.S. gigantus Cranopus, and the mysterious Grey Ogryns). Many within the department believe that many of these classifications are all separate types, and another revision of strain classification is required.
Ratlings (Homo Sapiens Minimus) are the smallest type of abhumans, serving as snipers in the Imperial Guard, although this role is becoming less popular now that pure human special weapon and light infantry squads are fielding marksmen as well. Ratlings also serve as cooks, a skill they no doubt learned in order to feed their own stomachs. They also tend towards kleptomania.
The Scalies of Necromunda are a relatively stable type of abhuman though not quite as stable as Ogryns for example. They are tall and broad reptilians with legendary strength and durability. Among the people of the Underhive they have a fearsome reputation.
Many worlds harbor an oppressed underclass of mutant workers, beings whose continued existence is justified to the Imperium only by ceaseless toil. Whatever quirk of genetics gave rise to such mutants will have been exacerbated by generations of backbreaking labour, exposure to hazardous environments, and filthy living conditions. The results are malformed creatures, deemed entirely impure by the authorities of the Imperium and fit only to serve as slaves. However, a certain level of natural selection is often at work in such populations. Thus, mutants, though misshapen, are tremendously tough and resilient.
It is not unknown for such mutants to be levied by the Munitorum to go to war and fight for the Imperium that hates them so. Drafting mutants is normally an act of desperation on the part of the Imperium, for the teachings of the Ecclesiarchy hold that it is the right and responsibility of the pure to fight the foes of Mankind, a tenet that certainly does not apply to mutants. A mutant force will often be accompanied by a great many Preachers, extolling the mutants to acts of sacrifice in order to atone for their sin of imperfection and attain a measure of the Emperor's forgiveness.
Squats (Homo Sapiens Rotundus) originated on high-gravity worlds towards the galactic core, making them short and tough. They seceded from the Imperium during the Age of Apostasy. They are said to have been mostly been wiped out by the Tyranids.
In the Imperium, there are mutants that, although of relatively stable genetic stock, are so misshapen and physically abhorrent that they are forced to live entirely separate from mainstream society. Hiding their swollen faces behind filthy shrouds, these creatures form an invisible underclass, forced to live in the unpopulated wilds of the Imperium's worlds and frequently the subject of brutal pogroms by zealous preachers keen to purge the impure. Such mutants are often forced into criminality and soon become easy prey for those who would whisper the lies of the Ruinous Powers. The Imperium has, on occasion, used such creatures as soldiers. However, they were treated as expendable and not expected to survive their first engagement. It's little wonder then that such beings turn to the Dread Powers, of Humanity considers these wretched creatures far less than human and has rejected them entirely.
Other recognized Abhumans within the Imperium are:
- Troths (Homo Sapiens Verdantus)
- Longshanks (Homo Sapiens Elongatus)
- Pelagers (Homo Sapiens Oceanus)
- Felinids (Homo Sapiens Hirsutus)
- Neandors (Homo Sapiens Hyannothus)
- 1: Inquisitor Resources: Inquisitor Rulebook, Part II, pg. 19
- 2: Necromunda: Outlanders Rulebook, pg. 25
- 3: Warhammer 40,000 6th Edition Rulebook, pg. 404
- 4: https://necromunda.com/gangs//
- 5: White Dwarf 303 (UK), "Chapter Approved", "Abhumans".pg. 40-45
- 6: Warhammer 40,000 9th Edition Rulebook pg.43
- 7: Liber Xenologis, pg.22-31
- White Dwarf 109 (UK), "The Imperial Guard"
- Codex: Imperial Guard (2nd Edition) by Rick Priestley, [Needs Citation]
- White Dwarf 110 (UK), "Ogryns" by Ivan Weeds and Graeme Davis