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 lizards 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.
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.
|Leader Organisms:||Broodlord - Hive Tyrant - Swarmlord - Tyranid Warrior|
|Small Creatures:||Gargoyle - Genestealer - Hormagaunt - Ripper - Termagaunt|
| Biovore - Hive Guard - Lictor - Neurothrope - Pyrovore |
Ravener - Tyrant Guard - Venomthrope - Zoanthrope
|Monstrous Creatures:|| Carnifex - Dimachaeron - Exocrine - Haruspex - Malanthrope |
Maleceptor - Mawloc - Tervigon - Toxicrene - Trygon - Tyrannofex
|Gargantuan Creatures:||Cerebore - Dactylis - Hierodule - Malefactor - Nautiloid - Viragon|
|Flying Creatures:||Harpy - Harridan - Hive Crone|
|Bio-Titans:||Dominatrix - Hierophant - Hydraphant - Viciator|
|Other Organisms:|| Bio-Weapons - Cortex Leech - Mieotic Spore - Mucolid Spore |
Mycetic Spore - Spore Mine - Sporocyst - Tyrannocyte - Zoats