Zoat
In early editions of Warhammer 40,000, Zoats were centaur-like creatures that were genetically engineered by the Tyranids as a slave race; half of all units in a Tyranid army were required to be Zoats. Because the Zoats were capable of using tools, capable of some form of rudimentary speech understandable by other races, and limited independent action, they were employed as an advance guard for the incoming Hive Fleets. Much like the Squats, they weren't very popular, and so had to be exterminated like the former: by getting nommed by the Tyranids. This supposedly happened after the Hive Mind of the hive fleets felt that Zoats were a liability due to their ability to think for themselves, and so the Zoats rebelled against the voracious army of hungry space bugs in an attempt to fight for their survival, which didn't go so well.
They aren't part of the current army and are nearly extinct due to either being killed by the Tyranids or Imperial expeditions and crusades, but they did get a shout-out in the Horus Heresy novels, when a Crusade Fleet mentions having encountered "centaur-like" creatures infiltrating the galaxy. And...are getting a mini for Blackstone Fortress.
In Warhammer Fantasy Battles, Zoats had the opposite personality - instead of being gregarious pilot-fish on the INCOMING! Tyranid Hive Fleets, they were solitary, mysterious, morally neutral, and rather peaceful druidic creatures. They were rarely seen to the point of being "semi-legendary" and the subject of endless scholastic debate. That worked in the earlier, RPG-influenced days of 3rd Edition, but they became incongruous with the gameplay of Warhammer when it became a strictly army list-based pure wargame, the zoats were quietly dropped along with the unpopular and too-grimdark for the kiddies Fimir and the redundant-with-dwarfs-and-halflings Gnomes. They returned, decades later, in Storm of Magic, as totally badass, four legged monsters that kill people with vines.
According to Tony Ackland, they were heavily influenced by the character of Adzel from Poul Anderson's Polesotechnic League stories.
A New start!
Zoats were just shown off at Las Vegas Open 2020. One of them, anyway - the alien will be in a Blackstone Fortress expansion, likely as a new adversary like the Dreaded Ambull, although we have no idea how they're going to retcon their survival. Though a fair guess, given how GW has given the Zoat a more "mechanically enhanced" look instead of using a mix of that and organic enhancements, it could be a case that GW retconned the Zoat's involvement with the Tyranids in their entirety, instead making them separate xenos race altogether.
According to the new info, no one in the modern Imperium has any idea where the Zoats came from, but, do know they are master manipulators who love self-cybernetic modification, have an obsession with ancient relics of any kind, and have a nasty habit of getting others to do what they want for them just to get them killed and leave the spoils ripe for the picking. The new figure for Blackstone Fortress is a character by the name of the Archivist who is on the station as a data-broker for Fortress raiders officially and as a team-killing adventurer unofficially. His sole goal is to loot as much archeotech he can possibly carry from the Fortress and book it to the next lawless hell-frontier to start it all again, leaving a trail of very peeved customers and "allies" in his wake.