The golden fleece is the fleece of a flying, winged ram named Crius Chrysomallos, or 'Golden-fleeced Ram', in Greek mythology.

 
It is best known from the story of Jason and the Argonauts, who were sent by Pelias, the ruler of Iolcos, to retrieve it from Aeëtes, the king of Colchis.
According to the Roman poet Ovid (43 BCE to 17 CE) in his Metamorphoses, Chrysomallos, the golden-fleeced ram, was the son of Poseidon, the god of the sea, and Theophane, the daughter of Bisaltes, a son of Helios and Gaia. Theophane was a beautiful woman, which meant she was often inundated with men. Poseidon carried her away to the island of Crumissa, where he transformed her into a ewe and himself into a ram. After laying with each other in ewe and ram form, Chrysomallos the golden-fleeced ram was born. The ram was sent by Nephele, a cloud nymph to save her children Helle and Phrixos from being sacrificed. He carried Phrixos all the way from Orchomenos in Boeotia to Colchis, but Helle fell off off the ram's back on the journey and plunged into the sea. When they reached Colchis, Phrixos sacrificed the Ram and laid its golden fleece in a sacred grove to Ares. This golden fleece would then become the object Jason and his Argonauts were sent to retrieve.
Chrysomallos, the Golden-fleeced Ram
Athamas was the king of a Boeotian city, Orchomenos in southeast Greece. He married Nephele, a cloud nymph, and they had two children, Phrixos and Helle. Athamas began to neglect Nephele and became enamoured with Ino, the daughter of Cadmus, the king of Thebes. Athamas deserted Nephele and married Ino, who held no love for her stepchildren. Ino was furious that Phrixos and Helle were Athamas' heirs, not her children with the king. So, Ino hatched a plan to dispose of Phrixos and Helle to clear the path for her children. Ino corrupted the realm's grain, which meant that it would not germinate when it was sown. In response to this sudden agricultural devastation, Athamas sent messengers to consult with the Oracle of Delphi to see what could be done. Ino however, intercepted the messengers and persuaded them to falsely report back to her husband that the way to resolve their grain problem was to sacrifice Phrixos (and in some sources, Helle, too).
Athamas was convinced he had to sacrifice his firstborn child (or children), but before they could be killed, Nephele sent the golden-winged ram to save her children. They were successfully whisked away from Athamas and Ino, but on their journey, Helle either became dizzy from looking down or lost her balance, and she plummeted into the sea. The place in which she fell became known as the Hellespont, or "Helle's Sea", and this area is now known as the Dardanelles. Phrixos survived the journey and safely disembarked the ram in the region of Colchis. He was welcomed by Aeëtes, the king of Colchis, son of Helios and brother to Circe and Pasiphaë. Next, depending on the source, Phrixos either sacrificed the ram or the ram itself gave his fleece to the boy before ascending into the stars to become the constellation Aries. Either way, Phrixos gave the golden fleece to Aeëtes, who hung it on an oak tree in a sacred grove to Ares and had it guarded by a large serpent, known as the Colchian Dragon, that never slept.