In Zoroastrian mythology, the three-headed dragon defeated by the hero Thraetona is Aži Dahāka.
Aži Dahāka is a monstrous dragon or serpent in the Avesta, especially in the mythic tradition surrounding the hero Thraetona, known in later Persian literature as Fereydun. The creature is associated with destruction, violence, and demonic power, and later texts give it three mouths and three heads.
Thraetona defeats and binds Aži Dahāka rather than simply killing him. In later Iranian tradition, the imprisoned monster is identified with Zahhāk, a tyrannical king whose shoulders bear serpents that demand human victims. The later story combines older Iranian myth with epic and literary developments.
Aži Dahāka should not be confused with the Simurgh, a benevolent or protective fantastic bird, or with Angra Mainyu, the destructive spirit. The spelling varies because the name is transliterated from Avestan and Middle Persian forms.