Centaurs were always known to be a wild and savage race, but out of the whole species, Chiron was wise and civilized.  One day, Hercules had a dispute with the centaurs, and shot them with his bow, unaware that his own mentor Chiron was among them, nailing Chiron in the foot.  Due to Chiron's immortal nature, he could never be released from the pain, so he traded his immortality with the titan Prometheus to finally take his eternal sleep, and was honored as the constellation Centaurus.