What is "zahhak"

Zahhāk or Zahāk (in /ذهّاک) or Bivar Asp ( meaning "he who owns ten thousands of horses") is an evil figure in Iranian mythology, evident in ancient Iranian folklore as Aži Dahāka (Azh dahak), the name by which he also appears in the texts of the Avesta. In Middle Persian he is called Dahāg or Bēvar-Asp, the latter meaning "[he who has] 10,000 horses". In Zoroastrianism, Zahak (going under the name Aži Dahāka) is considered the son of Angra Mainyu, the foe of Ahura Mazda.