The centaurs were a species of creatures from Greek mythology that resembled a horse's body with the head and neck replaced by what was basically a human body from the waist up, thus creating a bizarre human-animal hybrid. Unsurprisingly for a Greek myth, bestiality is involved … the centaurs being said to descend from an outcast who mated with wild mares and was himself the child of the debauched king Ixion and a duplicate of Hera made out of clouds. The majority of centaurs seem to have taken after their grandfather and been lustful, greedy, violent and typically drunk whenever they had the opportunity.
One exception to the rule of barbarian centaurs would seem to be Chiron - the wise philosopher, astrologer and sage who served as tutor to a series of Greek heroes. This exception may be due to the fact that he was not actually related to the other centaurs, with some accounts suggesting that he was fathered on a nymph by the titan Cronus in the form of a horse. The centaur Pholus is another possible exception and he also was said to have a variant ancestry to the common herd and both of these are sometimes depicted as being more like satyrs with horse parts rather than goat parts, possibly as a convention to indicate their greater humanity than most of their kind.
Modern adaptions tend to major on Chiron and Pholus and cut out a great deal of the savagery found in the original myths (much like the bowdlerization of the Satyrs) … there is also a tendency to use the term "tauric" to describe anything built along the same lines as a centaur with the top half of a human replacing the head and neck of an animal. This is a common RPG convention but ultimately a bad etymology - centaur means "bull killer" and the term tauric would actually mean "bull like". The Greeks recognised the onocentaur (donkey, rather than horse, bodied), but that was an end of the matter. It's also worth noting that for verisimilitude a centaur would probably need to be a carnivore (or at least an omnivore) given the difficulties in getting enough grass to maintain a horse's body through a human mouth.
Appropriately for their symbolic roots, all Greek centaurs seem to have been male1 - whether you treat them as a single gender species (like satyrs), use some form of bizarre sexual dimorphism to have them breed with some all female "species" (such as the sub-myth of satyrs reproducing with nymphs), or just introduce females (the modern and, arguably, sensible approach) is up to you (assuming you write the setting).
Game and Story Use
- Centaurs are usually bowdlerized as wise and good - surprise and amuse your PCs with original barbarian centaurs bent on rape and pillage.
- Theories about the origin of the centaur myths suggest they may result from early contact between proto-Greeks and horse nomads from central Asia - you may wish to use similar ideas to inspire actual (perhaps mythago?) species in your own campaign.
- Blend the cattle killing in with the noble savage/proud warrior race guy/barbarian tribe meme and go for some Zulu themed centaurs (perhaps human/zebra hybrids). Living out in the veldt herding cattle seems very centaur compatible.
- The opposite extreme might be a race of centaurs with destrier like lower bodies, massive humanoid upper bodies and full plate armour.
- The female centaur question … obviously the tribe of male only immortals is the most mythic idea and the mixed gender species the most sensible: which is preferable depends on the themes of the setting. The other potential option - the highly dimorphic species - is an interesting subversion; having the centaurs breed with mares would be the obvious option, with the offspring being horses if female and centaurs if male.
- Presumably the "bull killers" aren't going to get on well with minotaurs (an issue which didn't come up in the myths, there being only one minotaur who spent his entire life indoors on Crete and so never met any centaurs).