Once upon a time, long ago, while the creator Brahma slept, the demon called Hayagriva stole the Vedas.
Hayagriva hid the Vedas under the ocean so Lord Vishnu planned to take the form of a fish and save the Vedas from there.
During the next age or yuga, when Satyavratha Maharaja was ruling, Lord Vishnu appeared as a small fish and spoke to the king, "In seven days, the world will be destroyed. There will be very heavy rain and so I want you to save all the good creatures in the world like the sages and saints and those who have made it their duty to do good for others."
Sure enough, after seven days, there was very heavy rain and Lord Vishnu appeared again, but this time He came as a giant fish.
This was His Matsya avatara or form. He was golden in colour and had a great horn above his mouth, like a unicorn.
Lord Matsya brought with Him, a ship which could hold a lot of people. He saved all the sages and good people by placing them on this ship.
Then, diving into the depths of the ocean, Lord Matsya found Hayagriva guarding the Vedas which he had stolen. Lord Matsya killed Hayagriva and thus saved everyone from this demon and his evil ways.