As an attribute it is held in the right hand of many a fierce deity of Buddhism, especially the female ones, symbolizing critical, analytical and dissective wisdom, which cuts through and chops up all delusive appearances and reduces all things to voidness, suitable for the transformation into wisdom.
Here there are seven heads of Hayagriva arranged in vertical rows. Following the horse's head at the top is a single head of Hayagriva. Then follow two rows of three heads each. All of Hayagriva's heads have three eyes, bushy eyebrows and a five-skulled crown.
Hayagriva is worshipped in Tibet principally by horse-dealers because he is believed to frighten away demons by neighing like a horse. When invoked, he is said to announce his coming by neighing. The horse's head neighs loudly, and the sound is said to pierce all false appearances of substantiality, revealing the shining reality of freedom. His mantra contains the following verse:
"I request that you protect all horses. Increase the number of mares. For from the mother of the horse many supreme horses will be born. Please disperse the obstacles on the Path and reveal the right direction."
Hayagriva is a wrathful manifestation of Avalokiteshvara. There are believed to be 108 forms of Hayagriva. His special ability is to cure diseases, especially skin diseases even as serious as leprosy, which is said to be caused by the Nagas (malignant water spirits with serpent bodies).
This description by Dr. Shailendra Kumar Verma, Ph.D. His doctorate thesis being on the "Emergence and Evolution of the Buddha Image (from its inception to 8th century A.D).
Of Related Interest :
Hayagriva - The Protector of Horses
Ritual Implements in Tibetan Buddhism: A Symbolic Appraisal