The Centaur Chiron, a skilled archer and healer was just accidentally shot with an arrow poisoned from a hydra's blood.
Realizing that his end is near, Chiron treks to Prometheus, who was banished by the gods for giving man fire. Chained to a rock, each day an eagle pecked out his liver, and each night it would grow back. Chiron nobly asks the gods to trade places with Prometheus, saving him an eternity of pain.
The Gods reject Chiron's proposal, but the god Hera saw Chiron's act and Puts him into the stars as the constellation known as Sagittarius.