Sagittarius is the constellation of the centaur; a half man half horse creature.  While most centaurs were rude and violent, one centaur, Chiron, was different.  Educated by Apollo, and a skillful archer, Chiron became the teacher of many heroes, including Hercules.  One sad day, Hercules was attacked by a group of centaurs, and slew them all.  Unfortunately, Chiron was in the distance, minding his own business, but Hercules, not recognizing him, shot Chiron with a poison arrow.  Seeing his son's sadness, Zeus granted Chiron immortality by placing him in the stars.