Hayagriva is a wrathful manifestation of Lord Avalokiteshvara(The god of compassion). There are108 forms of Hayagriva.[Avalokiteshvara] His special ability is to cure diseases, especially skin diseases even as serious as leprosy, which is said to be caused by the Nāgas.
In Hinduism, one tradition identifies Hayagriva as a group of demons subdued by Vishnu, according to another tradition, Hayagriva was incarnation of Vishnu.
In Tibet, Hayagriva was promoted especially by Buddhist teacher Atisha.It is said that Tibetan horse-dealers worshipped Hayagriva because he could frighten away demons by neighing like a horse. When invoked he announces his coming by neighing, the sound of which pierces false appearances and disguise.

Hayagriva is one of the eight principal deities of Kagye where he is referred to as Lotus-like Speech. The instructions related to this form of Hayagriva are based on the so-called "three neighs of the horse"
Very wrathful, red in colour, with three faces, six hands and four legs, the right face is white and the left green. With three large round staring eyes and a cavernous mouth with sharp canine teeth and dark hair flowing upward. On top of the head are three small green horse heads. The right hands hold a vajra, trident and sword, and the left upper hand is outstretched in the mudra (sacred gesture) of bestowing protection, the remaining hands hold a spear and noose. Adorned with a crown of five dry skulls, earrings, gold and jewel ornaments, snakes entwined as jewelry and a tiger skin skirt, he is completely attired in wrathful charnel ground vestments and a set of vajra wings. The consort has one face and two hands, blue-black in colour, holding a skullcup in the left hand, adorned in wrathful attire and a leopard skin skirt. Standing with eight legs, the right bent and left straight, atop two corpses above a sun disc and multi-coloured lotus, Hayagriva dwells surrounded by the flames of pristine awareness.

In Hinduism

Hayagriva is an incarnation of Vishnu, depicted with a human body and a horse's head, brilliant white in color, with white garments, seated on a white lotus.
Hayagriva is a very important deity in the Vaishnava tradition. His blessings are sought when beginning study of both sacred and secular subjects. Special worship is conducted on the day of the full moon in August (Sravana-Paurnami) (his avatara-dina) and on Mahanavami, the ninth day of the Navaratri festival. He is also hailed as "Hayasirsa".
One of the legends states that during the creation, the demons Madhu and Kaithaba stole the Vedas from the creator Brahma, and to save them Vishnu took the form of Hayagreeva. This story represents the triumph of pure knowledge, guided by the divine over demonic forces of darkness.

Mantra of Haygriva