Story Lord Hayagriva - Less known form of Lord Vishnu

Brahmanda Puranam, Devi Puranam and Garuda Puranam describes  Lord Hayagreeva as an incarnation of Lord Vishnu .  Lord Hayagreeva is the  Horse faced Lord and Epitome of Intelligence.  He is the Guru of Goddess Saraswathy - Lord of Wisdom.  Chanting of hundred and eight names of Lord Hayagreeva  helps to activate the nerves in  the brain which perform the function of memory power .  Worship of  Lord Hayagreeva  needs utmost devotion, hygiene, dedication etc.. like worship of Sivalingam, Lord Narasimha and Anjaneya.   Lord Hayagreeva worshipped as Spatika Akruthi (shape).

Story of Lord Hayagrivam According to Devi Puranam - Shakti Purana

In the Sakta tradition of Hinduism, Hayagriva occupies a different role. Here, a demon named Hayagriva first appears as son of Kashyapa Prajapati. Through great penance the demon managed to obtain a boon from the Goddess Durga that he can only be killed by another "Hayagriva". This instilled a sense of invincibility and he started harassing the Devas. The Devas turned to Vishnu for aid, but despite a long struggle he was also unable to kill Hayagriva.

Tired and drained after the battle, Vishnu proceeded to Vaikuntha Dham to rejuvenate and meditate in padamaasna (a yoga posture) with his head supported by the upper end of his taut bow. The Devas once again approached Vishnu for help against Hayagriva but were unable to rouse him from his meditation. The Devas asked a swarm of termites to assist in waking Vishnu by gnawing away the string of the bow upon which he was resting. However, the snapping of the bow string produced such a resonant sound that the universe trembled, and the broken string lashed out with such force that Vishnu's head was severed from his body.

The Devas were mortified, and prayed to the goddess Durga for guidance. Durga was pleased with their prayers and told the Devas that they need not fear, as no incident in this universe was without some purpose. She then told them of her boon to Hayagriva and asked them to attach the head of a horse to Vishnu so that he in the guise of "Hayagriva" (that is, one with a horse's neck) could kill his foe.

Brahma attached the head of a white horse to Vishnu's body and the revived Vishnu entered into battle with Hayagriva and eventually killed him.

According to some Vishnu and Brahma puranas

Another legend has it that during the creation, the demons Madhu and Kaitabha stole the Vedas from Brahma, and Vishnu then took the Hayagriva form to recover them. The two bodies of Madhu and Kaitabh disintegrated into 2 times 6 — which is twelve pieces (two heads, two torsos, four arms and four legs). These are considered to represent the twelve seismic plates of the Earth. Yet another legend has it that during the creation, Vishnu compiled the Vedas in the Hayagriva form.

Another Version of Same story was - When the destruction of the earth was going on and then a demon named HayaGreeva was memorizing all the vedas to become immortal - then Lord Vishnu cut his head where all the Vedas are stored and he himself took that head had with him safely and returned to  Brahma to continue the Creation -

Some consider Hayagriva to be one of the Dashavatars of Vishnu.

He along with Krishna, Rama and Narasimha is considered to be an important avatar of Vishnu.

In Pancha mukhi Hanuman (five Head)one of the five heads is considered to be Hayagriva( some times shown as horse head and sometimes shown as normal human head.

Lord Hayagreev's Sannithanam is there in Srirangam Temple. Hayagreev in Srirangam is very famous for children's education. The sannithanam is very beautiful and all rituals done according to vedic principles very strictly.