Kinebrach

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 basics

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). 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.

Kinebranch technology and its possible Chaos ties

Most sources say the Kinebranch 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? 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), the warpsword that made possible Horus's corruption into Chaos. That could be seen as a take that to the Imperium after they eradicate both the Interex and them, but considering the species's ties with warp stuff, it's probably they are being tortured in the Warp as we speak. The blade existed before the Imperium showed up, so it is not a "take that" at the Imperium. Also, it is a Nurgle-aligned Chaos artifact, which could have only been made by either being aligned with Chaos or using Chaos. Which is the same thing since it is Chaos and so if you use it yet think you are in control of yourself, you are already lost. They even have a superweapon buried somewhere in the Sabbat Worlds that was so terrible that even dedicated Chaos officers would rather it was destroyed than ever risk it being used (though to be fair the Blood Pact is as sane and pragmatic as it gets for a faction who worships the god of nihilistic slaughter).