They were two fearsome bulls which breathed fire; they were created by the god Hephaestus and were gifted to King Aeetes of Colchis. When Jason reached Colchis, Aeetes promised him the Golden Fleece if he could pass some trials. One of them was to yoke the Khalkotauroi and plough a field with them. He should then sow it with dragon's teeth. Medea, Aeetes' daughter, had fallen in love with Jason, and offered her assistance; she gave him a potion which protected him from the fires of the Khalkotauroi. He then yoked them and passed the task he was given.