With the lower body of a horse and torso of a human, centaurs are one of the better-known figures from Greek myth. Likely originating from tales of first contact with a culture that had either domesticated horses for riding (which early Hellenic society had not) or worshipped them, centaurs were often depicted as wild, uncouth and drunken thugs.
In the Inferno, centaurs guard the violent - murderers, tyrants, kings and warlords - in the Seventh Circle. The sinners they watch over are made to stand in the Phlegethon, a river of boiling blood, up to a depth that suits their crimes. The centaurs ride along the banks shooting arrows at souls who try to leave or change position. Virgil and Dante meet Chiron (who according to popular myth was the tutor of Achilles, unrelated to the other centaurs and therefore known for his nobility and wisdom; in the Comedy he is perhaps only slightly less aggressive than his fellows). He offers to have another centaur, Nessus, help Dante and Virgil cross the river of blood.
Cacus, another, more monstrous centaur, appears later in the Eighth Circle among the thieves, with snakes emerging from his back and a dragon riding on his shoulder. This is another character drawn from the Aeneid; interestingly though, the earlier poem adheres to the original myth that depicts Cacus as a fire-breathing, man-eating giant, son of the god Hephaestus. The Aeneid has Cacus living in a cave on the site that would later become Rome. Stealing eight of the cattle that Heracles had himself stolen from Geryon, Cacus was tracked down and killed by the demi-god. Cacus' crimes in the Divine Comedy - the reason for his separation from the other centaurs - are identical to those in the source text, but the change in iconography seems to be Dante's own invention. This proved to be an enduring trope though - three hundred years later, Cervantes references Cacus as a centaur in the prologue to Don Quixote.