Centaurs are mutated creatures. They are a by-product of infection by the Forced Evolutionary Virus.
Centaurs encountered in the Core Region and Mojave Wasteland were created by the Master. The creation process involved tossing a varied mix of humans, dogs, and other types of animals into a Forced Evolutionary Virus (FEV) vat to be infected by the virus, and then see what mutant chimera emerged as the virus produced rapid and unpredictable mutations. The result, centaurs can differ quite radically in physical appearance from one another. However, all known centaurs are somewhat humanoid in appearance.
Like their western counterparts, the Capital Wasteland centaurs are created by infection with the FEV along with the super mutants. Their birthplace, like the Super Mutants, in Vault 87.
Centaurs of all known varieties are not harmed by radiaton, but seem drawn to it, as they are often found in heavily irradiated environments.
Physically, these centaurs are around the size of a bear, with a large, misshapen torso and six humanoid arms that serve as legs. A cluster of wriggling fleshy tentacles bloom out from their underside, scrabbling at the ground around them. Branching from their front halves are a pair of long, thick necks, with a canine head on the right and a humanoid one on the left. The canine head has a thick metal collar around its neck, adorned with spikes, not unlike an ordinary dog collar.
So far, the two-headed centaurs have only been encountered on the West Coast, in the Core Region.
[http://fallout.wikia.com/wiki/Centaur_%28Fallout:_New_Vegas%29 ]In this type of centaur, only the human-like head remains, and the feelers on the back are replaced by three long, tentacle-like tongues. Unlike its two-headed "cousins", this kind of centaur uses a ranged attack: a high-arcing, slow-moving saliva projectile that can carry lethal doses of radiation. Oddly enough, it is capable of igniting lingering gas vapors as well. In close quarters it utilizes a long, three-pronged tongue to whip and slash at any intruders.
This kind of centaur can be encountered both in the West and the East of the former United States.
 As the name suggests, they are a bigger and stronger variant of a centaur. having been exposed to much more radiation than regular centaurs, they have significantly more muscle mass as well as thick clusters of tumors growing from their backs.
An even bigger variety of the Evolved Centaur, the Giant Evolved Centaur is the largest and most dangerous variation of Centaur to be discovered to date.