The myth of Hayagriva- man with a horse neck (griva), is mentioned in Maitreyi Samhita ( 4.9.1). It explains why Lord Vishnu’s head had to be substituted with a horse head.

In the Vedas, Vishnu is the god of sacrifices and is identified as ‘yajna’. Once  Agni, Indra, Vayu, and Yajna ( Vishnu) decided to perform a sacrifice. Gods decided that whoever receives a sacrificial gift first will divide it equally among the rest of them. Yajna (Vishnu) received the gift first. But he got greedy and refused to share it with others and ran away from the sacrificial grounds. The other gods immediately chased him. But Yajna who was quick-footed outran the gods and disappeared in the wilderness. After a while when he noticed that gods were nowhere in sight, Yajna was tired and decided to take a quick nap. But he did not want to take any chances as gods would catch him anytime, so he planted one end of his arched bow on the ground rested his head on the other end, in readiness to shoot an arrow,  and dozed off.

Meanwhile, the gods after searching high and low finally found Yajna ( Vishnu) fast asleep with his head rested on his bow. They decided to cut his head off. But who would do the task?  They commanded the ants in the wilderness to come together and gnaw at the bowstring of Yagna’s bow. But the ants refused to oblige and the Gods made a deal that ants would from then on get a part of sacrificial oblation in the form of clarified butter (ghee). Tempted by the offer ants agreed and quickly gnawed off the string. Released from the tension of the bowstring the arched bow snapped and flung the head of Yajna/ Vishnu high into the air decapitating him. The head rose into the sky and turned into a bright golden ball of fire and became the Sun. The Gods then replaced the head of Vishnu with a horse head, and thus Hayagriva was born.

In another version of the myth found in Devi Bhagavat (1.5) it is said that Devi had blessed the rakshasa Hayagriva- (who was born with a horse head) with a boon that only a divine being bearing the name Hayagriva would be able to annihilate him. Hayagriva knew that the chance of another Hayagriva existing on the earth was impossible and became convinced that he had become immortal.
With the knowledge of his immortality, he grew arrogant and stole the Vedas from the earth. Since Vedas contained all knowledge/light, earth soon plunged into darkness. The Gods convened and requested Vishnu to retrieve Vedas and save the earth. Vishnu soon went to Hayagriva’s abode and found him asleep on the bow strung with an arrow ready to be shot. He requested the ants to gnaw the bowstrings. Ants obliged Vishnu and thus the bowstring was severed. The arrow shot past cutting both Haygriva as well as Vishnu’s head at the same time. Vishu’s head flung into the air and turned in to Sun. Hayagriva’s head was later placed on Vishnu’s head and thus was given the name Hayagriva.

——————————-

Story Collected by : Vidya Kamat
Text Source: Bhartiya Sanskriti Kosh, ed. Mahadeo Shashtri, see under Hayagriva
Location: Pan India
Image Source: Wikipedia