The Kroot are a minor species featured in Warhammer 40k, who serve a supporting role for the Tau similar to that served by the Ratlings or Ogryns for the Empire. The Kroot fight as mercenaries (called "Carnivores"; read below for why) in Tau armies, handling close combat engagements for the relatively frail Tau.
The Kroot are fairly frightening looking creatures. They allegedly evolved from an avian life form, according to the Games Workshop literature, but appear more like a reptile of some sort (of course, the two are related). They have large, bulldoggish jaws filled with sharp teeth, clawed hands and feet, a slightly bow-legged posture, and a mass of sharp spines or quills protruding from the backs of their heads. Their skin is generally hairless, and varies widely in coloring as a result of their variable genetics and a limited camouflage ability.
The Kroot are a genetic marvel. They have a highly variable gene structure, that results in an ability to go through rapid evolutionary changes. Even more bizarrely, they are able to assimilate genetic information by ingesting the flesh of other species. Having defeated an enemy, they will often stop in the middle of a pitched battle to chow down on a particularly good looking specimen. Unknown organs in their digestive system analyze the DNA they ingest, and add promising looking bits to the DNA of the carnivore doing the eating. (Alright, alright. I know. But it wouldn't be science fiction if it was going to show up in Nature any time soon!) By this process, the Kroot are able to add the strengths and abilities of their defeated enemies to their repertoire. Kroot are voracious eaters, and hunt constantly. Studies by the Tau of the Kroot home worlds have revealed that the Kroot have already assimilated nearly all the promising DNA from the native species of the worlds that they occupy.
All this DNA swapping wackiness does have its downside. Certain groups of Kroot, in an attempt to acquire a few too many traits from a given species, have mutated into sub-species incapable of higher thought or levels of organization. The two most well known of these are the Kroot hound and the Krootox. The Kroot hound was apparently an attempt to acquire the characteristics of an efficient pack hunter. This subspecies is quadrupedal, non-intelligent, and vicious. They are used as fast attack troops, and to harry and pursue retreating foes by the Kroot.
The Krootox seems to be an attempt to gain the positive characteristics of some sort of grazing herd animal(!). Maybe it was just a case of over-eating; the multiverse may never know. In any case, these large, slow, small-brained creatures are ridden into battle by their smaller and less mutated brethren, who use them as weapons platforms for heavy weaponry. Yes folks, you read that right the first time.
In an attempt to prevent these sorts of things from happening too often, Kroot are often lead into battle by Shapers, more intelligent Kroot who basically direct their brethren as to what they should and should not eat, and in what quantities. This is what holding public office gets you in the grim world of the future.
Politically, the Kroot are one of the many species wholly incorporated into the Tau empire. The Tau trade with the Kroot, giving them weapons, transportation off planet, and the opportunity to eat new and interesting species. The Kroot, in exchange, fight loyally and fearlessly for their overlords. Their level of technological development is not high (they fight with projectile rifles that have been slightly upgraded for them by the Tau), but their willingness to enter close combat, their tactical abilities, and their knack for infiltrating into difficult terrain (they are particularly effective in the jungle, due to their arboreal descent) makes them invaluable to the Tau. Kroot are often viewed as a little distasteful (needles to say, no one has them over for dinner parties), but are respected for their devotion to the Greater Good, and so are accorded full rights and citizenship in Tau society. Which really isn't a bad deal for blood-thirsty, cannibalistic savages.