The Golden Fleece of Greek mythology was the treasure sought by Jason and the Argonauts.
The fleece itself came from a golden ram, sent to rescue Phrixus and Helle, the children of Athamas and the goddess Nephele. Phrixus' step-mother, Ino had plotted to have him sacrificed in a fit of jealousy. The ram carried Phrixus safely to Colchis, although Helle fell into the Hellespont (which was named after her).
Once at Colchis, Phrixus married the daughter of King Aeëtes and sacrified the ram, presenting the fleece to the king, who hung it from an oak tree. There it stayed, until stolen by Jason.