Rainbow

Rainbow is a Hippocampus. He befriends Tyson in The Sea of Monsters.He is bigger than most of his race because he is meant to carry Tyson.

The Sea of Monsters
Rainbow is a half Horse-half Fish or Hippocampi (Fish Ponies as Tyson calls them.) Rainbow appeared with two other hippocampi at the beginning of the quest he took Tyson to the Princess Andromeda, Luke's ship, while his friends took Percy and  Annabeth. He followed them to the Sea of Monsters and rescued Tyson from the CSS Birmingham when it exploded. He took Tyson to Polyphemus' Island after Clarisse's ship explodes. Shortly after he rescued Percy and his companions when the Queen Anne's Revenge (Blackbeard's ship) sank and Rainbow (and friends) took them to Miami. After the quest Tyson rides him to Poseidon's forges. Tyson was the one who named him Rainbow.

The Last Olympian
Rainbow is later seen in The Last Olympian, after Percy called him near the East River, wanting to know if Tyson is all right, and when he learns about Tyson becoming a general he asked if he would still give him cupcakes. It is possible that he became Tyson's mount.