The Kinebrach was a xenos species that populated multiple systems near the Interex empire on the Segmentum Pacificus during the times of the Great Crusade. They were basically intelligent gorillas IN SPACE.
The Kinebrach was a species of humanoid aliens similar to the Earth gorillas. Before humanity started conquering the stars, the Kinebrach had a pretty considerable empire of their own, but by the time the Interex arrived there, their empire was on the way out. When confronted with the Interex humans, they decided to ally themselves with it and become a subservient species, rather than try to fight this rising power (like how the Dark Age humans got on with aliens until they betrayed us when we were weak, isn't it great the Interex learned not to repeat that mista-oh wait). In exchange for peace and technology exchanges, the Kinebrach were forbidden to carry arms, save in times of war. When in war, they usually aided the Interex as shock troops, due to them being big space gorillas and all. This species, like the Interex themselves, went the way of the Dodo once they made contact with the Imperium of Man. By the end of the Horus Heresy, the species is considered extinct; that being said, enclaves might still exist.
Kinebrach technology and its possible Chaos ties
Most sources say the Kinebrach developed a very complex and advanced technology, further advanced by its combination with Interex tech. In particular, the Imperium were extremely wary of their weaponry due to the suspicions of them using Warp stuff (or even Chaos stuff) to make them deadlier. There were sayings that they were able to create poisons and weapons specifically made to kill a target by whispering them the name of their victim, which sounds like warp-fuckery. This is very interesting, since the Interex were very adamant in their fight against Chaos (or Kaos, as they called it), so why didn't they recognise these practices for what they were? They did, and saved the Kinebrach from Nurgle-induced lethargy and stagnation, but really should have thrown their tech away. The most important piece of tech the Kinebrach ever made was the Anathame (surprisingly similar to the big E's nickname "the Anathema" except the "e" and "a" were swapped, because it is a play on that and "athame"/ritual knife), the warpsword that made possible Horus's corruption into Chaos. In other words, along with this fucker, some space orangutans are responsible for the current state of affairs. Great.