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 in appearance to the Earth gorilla. Before humanity started spreading out among 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, the two species would engage in a brief conflict before they eventually decided to ally with each other. In exchange for peace and technology exchanges, the Kinebrach (along with their human allies) 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 how deadly they were; its possible that there might have been some Warp magic going on here, but it's never confirmed- not that the Imperium would have had any clue regardless, thanks to the Imperial "truth". 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 weapons for what they were? Maybe because not all Warp tech = Chaos; the Imperail army itself uses weapons that utilities the warp. The most famous 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), was stolen by Erebus and given to a Champion of Nurgle, who then used the warpsword to mortally wound Horus and left him comatose. Erebus would then manipulate his desperate sons by leading them to a Chaos cult who would then perform a ritual that corrupted him completely. In other words, Fuck this fucker