Lord Vishnu once declared that whenever there is any danger on the earth, he would adopt form and come down on earth to save its people.

One day, when Brahma was sleeping, Hayagriva, a horse-headed demon, stole the holy Vedas from under Brahma's head. He then ran away and hid in the depths of the ocean. Brahma had to read the holy books to create the universe. Since he was unable to do so now, he was very disturbed and approached Vishnu for help.

Vishnu took the form of a fish and dived into the ocean to get back the Vedas from the terrible demon. There was a fierce fight between Vishnu and Hayagriva, which went on and on. Finally, Vishnu killed the demon and brought the holy books back to Brahma. Brahma was very thankful  and was able to resume reading the Vedas.