Matsya

Matsya is an avatar or incarnation of Lord Vishnu. It is the first of the 10 major incarnations (Dashavatar) of Lord Vishnu.

The Legend of Manu
Manu was a king in ancient India. One day, when Manu was in his morning prayer, he found a tiny fish in the water which was brought for his ablution. The tiny fish pleaded to him to save him, or else the other larger fish would devour him in the sea. In return, the fish promises to save him from an incoming flood. Manu agrees and puts the fish in a pot of water. The next day, the first grew so big that it couldn't be contained in the pot. So, he transfers the fish to a well filled with water where it can grow freely. The next day, again, the fish had grown to fill the well. Left with no other options, he lets the fish into the sea as it is big enough to survive on its own. The fish thanks Manu, and tells him when the great floods will commence and also reveals that he is Lord Vishnu.

The fish also tells him to build a ship and fill it with all species of plants, animals, and also the seven sages (Saptarishi) to escape from this disaster. On the predicted day, Manu comes to the sea with his built ship. The fish which had grown to an enormous size now, asks him to tie the ship to its horn with the help of the great serpent, Vasuki. The boat escapes from the flood to the Himalayan Mountains where Manu, the only human survivor performs yajnas. The goddess Ida appears from the yajna and tutors him in Puranas and Vedas for the upcoming age.They establish the human race, 'The Race Of Manu'. It is believed that the word man originated from 'Manu'.

Slaying of Hayagriva
At the end of Kalpa (time), Hayagriva, a demon steals the Vedas from Lord Brahma, the creator, that escaped from his yawn. Lord Vishnu notices this theft. As there was a pralaya (end of the world) approaching, he has to save the world from it. He goes to Manu and helps him along all the creatures in the boat to escape. Then, he leaves to restore the stolen Vedas. He goes to Patala (underworld) and slays Hayagriva. Then, he safely restores the Vedas to Lord Brahma, as he can start the creation of the world.

Thus, Matsya restores peace in the world, fulfilling his avatar's motive.

Etymology
Matsya is derived from 'matsya' which means fish in Sanskrit. It also means 'the joyous one'