Centaurs are the result of combining various kinds of animals and people and infecting them with the Forced Evolutionary Virus. The result of this created horribly disfigured creatures with little intelligence and an affinity for their super mutant masters.
The centaurs encountered in the Capital Wasteland have a human head and three long, tentacle-like tongues. Like their western counterparts, they are created by infection with the FEV along with the super mutants. Their "birthplace", like the super mutants, is Vault 87.
They lack feet, which are substituted with human arms which confines them to a slow, shuffling type of movement.
It is very likely that centaurs were once human. This is based on concept art and Vault 87 logs.
Centaurs can pose, at best, a moderate threat level to players but are generally a weak enemy. Their weak spot is their long, tentacle-like tongues which can be easily targeted in V.A.T.S.. Usually they don't carry anything, although they sometimes have chems like jet, small amounts of bottle caps, and rarely random items like teddy bears, leather belts, pass cards, or a bobby pin or two.
They spit slow moving, radioactive projectiles as a ranged attack which may ignite nearby gas leaks, but is relatively easy to dodge. The damage done by the spit itself is low, but can increase your rads quickly (+5 per second for 15 seconds). When up close, the centaur whips its tongue at you. This melee attack is rather delayed, much slower than the player's melee attacks. Centaurs are rarely found wandering alone, so they may indicate that super mutants are nearby. They most often try to use their radioactive spit against the player, but will use their tongue if you get close.
The basic centaur variant in Fallout 3. They are slow moving and often travel with a super mutant. Centaurs have both a long range and a close range attack. Centaurs are not a major threat, however they may cause trouble for low level players.
The following is based on Fallout 3 cut content and has not been confirmed by canon sources.
Clancy was a named centaur cut from the final release of the game. Although identical to normal centaurs stat-wise, Clancy's internal name associates him with the quest Wasteland Survival Guide. Although he cannot be found anywhere in the game, Clancy can be found in the game files. His reference id is 2d3be.