Face Eater said:
Extra nerd points if you can include the equivalent of any traditional fantasy races with appropriately sci-fi name.
I'll rise to that challenge!
The dividing line between mutant and abhuman is a thin one. After all, most abhuman populations began as mutants, but have been accepted by the Imperium as having some unusual attribute or skill which makes them useful to humanity as a whole. Those abhuman populations tolerated by the Imperium tend to breed true, displaying little variation or prospect of further mutation. This distinguishes them from the hordes of shuffling, disease-ridden mutants who form a despised underclass in many Imperial communities, their freakish and disturbing appearance a mute testament to the polluted and radioactive nature of most Imperial slums.
Broadly speaking, the safest definition of abhuman is "a human whose genetic template deviates from the homo sapien norm in a fashion expressly or tacitly tolerated by the Imperium."
Sometimes the degree of variation can be tiny, or purely cosmetic. On other occasions tolerated abhumans can be wildly different to a "typical" human, if such could be said to exist in the vast and diverse Imperium of Man.
The inhabitants of Hwurk, a tiny frontier moon in the Segmentum Obscuras, are an example of how a tiny genetic variation can affect the prospects of an entire world's population.
Inhabited by the descendants of a small vessel that crashed on the verdant moon at some point during the Dark age of Technology, Hwurk is a low-grav world covered in forests of vast size, with trees that soar hundreds of metres into the azure skies. The planet possess an aggressive ecosystem of large predators, though it cannot be said to be as deadly as a true death world.
When it was rediscovered by the Imperium in M37, the inhabitants had regressed to a stone-age culture who worshipped the site of their craft's first landing on the world. Genetic tests upon the population showed that the inhabitants had suffered a significant degree of genetic "bottlenecking," suggesting that the thousands of modern Hwurkans were all descendants of a very small group of initial crash survivors, perhaps as few as 100 in total.
One of the survivors carried an unusual gene: this nameless progenitor suffered from a condition known as hypertrichosis. Hypertrichosis causes unrestricted hair growth across the entire body in both sexes. Approximately 25% of the Hwurkan population bear rich, brunette hair covering every inch of their bodies. This is typically left to grow out. Combined with their stature - their low gravity world and low initial genetic diversity has led many Hwurkans to exceed 7 feet in height - this hair growth results in an unusual, inhuman appearance that many within the Imperium find disturbing. In fact, Hwurkans are entirely human beneath their long hair, and are loyal and industrious servants of the God Emperor.
As such, the Hwurkans routinely found Imperial Guard regiments, a significant number of which contain platoons of tall, hairy abhumans, derisively known as "Hwurkies" by other Guardsmen. These guardsmen are typically used as terror troops to cow rebellious human populations; their height and full body hair gives them a feral and bestial appearance, though in fact they are known for their civilised and cultured nature. Hwurkies commonly report for these duties naked (in order to enhance their terrifying appearance) apart from bandoliers of lasgun ammunitition, and are a striking and famous feature of the Segmentu Obscuras Imperial Guard.