The story of Sagittarius:

This sign is representative of the centaur, Cheiron. Cheiron was a friend of many great hero's in Greek mythology such as Achilles and Heracles. While hunting, Heracles accidentally shot Cheiron in the leg with a poison arrow. Cheiron was immortal so he couldn't die, he just had to take the unending pain. Heracles promised to help him somehow. Upon his ventures, Heracles came upon Prometheus who was trapped with no way of escape. The only way Prometheus could be set free was for someone else to take his place. Cheiron wanted only to be relieved of his insufferable pain, so he took Prometheus' place and died. In honor of the noble act, Zeus placed Cheiron amongst the stars.