Wikipedia
Hayagriva, also spelt Hayagreeva ( Sanskrit: , IAST: ), is a horse-headed avatar of the Lord Vishnu in Hinduism. In Sanskrit, Hayagrīva means haya=Horse, grīva=Neck(Face?).
In Tibetan and Japanese Buddhism, Hayagrīva ("having the neck of a horse") is an important deity who originated as a yaksha attendant of Avalokiteśvara or Guanyin Bodhisattva in India. Appearing in the Vedas as two separate deities, he was assimilated into the ritual worship of early Buddhism and eventually was identified as a Wisdom King in Vajrayana Buddhism.
In Tibet, Hayagriva was promoted especially by Buddhist teacher Atiśa and appeared as a worldly dharmapala. His special ability is to cure diseases, especially skin diseases even as serious as leprosy, which is said to be caused by nāgas.
In Japanese Mahayana Buddhism, Hayagriva was evaluated as a Avalokiteśvara of the guardian deity for horses. The horse was symbolized as a vehicle, not as one of Hayagriva's heads.