Zoats were a centauroid alien race, introduced in the original Warhammer 40,000 Rogue Trader rulebook as a slave race of the Tyranids, and were available as mercenaries in Eldar pirate groups. They appeared only in First Edition.
Inspiration and etymology
Zoats were introduced in Warhammer 40,000's first edition rulebook as a part of the Tyranid army. They are described as one of the Tyranids' genetically engineered slave races, and fulfilled for the Tyranids the role of warrior. In game terms, any Tyranid force had to have at least half its numbers made up by Zoats. Although naturally a psychic race, they could not develop psychically under Tyranid dominance.
Zoats were not popular and did not survive into the game's second edition.
In 3rd edition's background, the Zoats were revisited, if only in a background story about their extinction.
Once the most common of the Tyranids' many specially engineered slave races, Zoats were unlike most of the Tyranids' assimilated worker-creatures, having independent thinking, and were therefore often used by Tyranids as a type of ambassador. Hence it was Zoats that were the first members of the Tyranid race to make contact with the Imperium of Man. Retrospective accounts talk of a race, assumed to be Zoats, having first made contact with the Imperium as early as M38 as their fleet of semi-organic spacecraft attempted to enter Imperial Space, claiming they were escaping slavish oppression. It is clear that via their initial contact the Zoats did not declare their true intentions, nor the true intentions of their masters.
Alternatively, it is possible that as the Hive Fleets evolved more, the Zoats became a less useful tool, their free thoughts an undesired quality. Many of the Zoats, sensing that they were soon to reach the end of their helpfulness to their brutal masters, staged a rebellion. It quickly was crushed, and the rebel Zoats were soon purged from the Hive Mind's forces. Those who survived fled ahead of the main Hive Fleets, and made up that first fleet, and were soon trapped between their former masters and their new aggressors who saw them as an invading force. Being warlike by nature, they typically reacted in kind, and so ultimately allied themselves against the Imperium regardless.
Eventually, as Tyranids became a more common sight in the galaxy after the invasion of Hive Fleet Behemoth, Zoats were no longer seen outside the ranks of the Tyranids. Their sightings became less and less frequent, and they appear to have been entirely absent from Hive Fleet Leviathan. If the only Zoats to have entered the Galaxy were those of the initial fleet, known as Colossus, then their absence can be explained, as the last known vessel of that fleet was destroyed above Zorastra in 226.M39, after the fleet provoked the Zorastra-Attila war.
Physiology and biology
Zoats were built for combat, having a more centaur-like appearance then the rest of the Tyranids, standing on four legs with their two fore-limbs acting as stocky arms. Zoats also differ from other Tyranids physically, as they do not have an exoskeleton but instead have thick scaly skin as bodily protection. Overall they stand taller than a man, and are extremely muscular, being more than a match for most in combat. They are also proficient in ranged combat, and have access to a number of weapons and are equally deadly with each. One weakness they do suffer is their inability to breathe all but the most specific atmospheres; this is the reason that Zoats almost always are seen wearing respiratory masks.
Zoats have highly-developed brains, meaning they do not require the control of the Hive Mind, and are therefore capable of travelling far beyond its range. Nevertheless, like Tyranids, Zoats have strong inherent psychic powers, chiefly telepathy. Their powers differ from other Tyranids, though, as they are capable of communicating to members of other races via their telepathy. They seem incapable of communicating verbally.
Zoat technology seems to be a very unusual cross between symbiotic, organic devices and entirely mechanical devices. They are able to both carry living weapons, such as the Fleshborer and Bonesword, and more standard mechanical weapons, such as the flamer and missile launcher at the same time. Their spacecraft also displays this odd mixture, being partially organic, in that they are conch-like and grown, but also inanimate, in that they are formed from stone rather than the bone-like material Tyranid ships are made from.
A possible reason for this is their independence from the Tyranid race as a whole and need to be able to interact with the universe around them without the influence of the Hive Mind. Either way, their dependence on non-organic technology is now complete, as all Zoats appear to have to wear respirator masks.