Kroot
In the Warhammer 40,000 universe, the Kroot are a species of humanoids that evolved from avian creatures. Kroot are tall aliens (a good half-meter taller than most humans), their avian ancestry giving them a bird-like beak and long quills protruding from their heads like hair. Smaller quills, possibly the evolutionary remnants of feathers, can be found scattered over their bodies.
A unique feature of the Kroot is that they evolve by selecting traits of their defeated foes to absorb by eating them. Due to this, the many Kroot warbands across the galaxy often look radically different. Kroot leaders are known as Shapers; they are generally the Kroot with the best ability for recognising desirable traits in defeated foes and tell his Kindred (the word for a Kroot clan) what to eat to suit the task at hand. For instance, a Shaper who wants his Kroot to gain muscle mass will take on campaigns against Orks to acquire the right DNA, while extensive feeding on flying predators will allow the Kroot to grow wings. This eating of their foes ties in with the religious beliefs of the Kroot: they believe that when a warrior of any race dies, his warrior spirit should be kept, and the only way to do this is by eating his flesh. The Kroot practice this in their "burial" customs, in which the body of the deceased is consumed by the kindred.
Kroot who prey extensively on a particular species will begin to take the characteristics of that creature. In sentient species such as Orks and Humans, they may also take on cultural aspects of that race as well. Kroot who have fed on Dark Eldar, for example, soon begin to show signs of the cunning and intense cruelty the Dark Eldar are known for.
In the case of animals, this sometimes leads to evolutionary dead ends. A Kindred will find itself locked into an animalistic and unintelligent form, unable to evolve themselves any more. Species such as Tyranids who also absorb the DNA of their prey are particularly dangerous for Kroot to prey upon, as they may acquire too many undesirable Tyranid characteristics. The ecology of the Kroot homeworld of Pech includes many species of animal that are partially Kroot, the results of a kindred's evolutionary missteps. The Kroot use these related creatures as laborers, pack animals, and even as weapons. The most common examples are the canine Kroot Hounds, the lumbering and powerful ape-like Krootox, the dromaeosaurid Knarloc riding beats, and the massive dinosaur-like Great Knarlocs. Another creature of note is the Kroothawk, an animal held sacred by the Kroot, possibly because it may be their evolutionary ancestor. There are other Kroot sub-species that are considered outcasts in Kroot society and are killed by Kroot if found.
In order to gather genetic material from all over the galaxy, the Kroot offer themselves as mercenaries, and sell their services to anyone willing to pay them. They travel the galaxy taking limited contracts from both major and minor races (most of which are too small to be included in the game), and are regularly employed by the Tau. While the contract with the Tau is supposed to be an exclusive one, the Kroot regularly send out their warspheres to continue their mercenary trade amongst the wider cosmos.
Technology
Outwardly, the Kroot seem to be a simple pre-industrial society, seemingly incapable of achieving (powered) flight, much less traveling through space. However, Tau observers on Pech have noted evidence that may point to an industrial base hidden on or even inside of some mountains. It is probable that the Kroot deliberately keep their general level of technology low, using it only for weapons and interstellar travel, in order to keep their species strong.
The primary weapon deployed by the Kroot is a rifle whose design is derived from the fighting staffs used by their ancestors. This weapon is fitted with blades on the stock and the end of the barrel, which the Kroot make deadly use of in close combat. Originally a slug-thrower, the weapon was modified by the Tau to fire a charged particle similar to their own Pulse Rifles. This modification quickly became standard for all Kroot.