Rainbow is half-horse, half-fish, or a Hippocampus ("Fish Ponies," as Tyson calls them). Rainbow appears with two other hippocampi at the beginning of the quest, as a gift from Percy and Tyson's father, Poseidon. They take Tyson, Annabeth Chase, and Percy Jackson to the Princess Andromeda, Luke Castellan's ship. Rainbow is sad to leave his new friend, and performs backflips with Tyson on his back trying to get him to stay.
When that doesn't work he follows them to the Sea of Monsters, and rescues Tyson from the CSS Birmingham when it explodes. He takes Tyson and searches the sea for Percy and Annabeth who had gone off to recover the Golden Fleece, finally finding them at Polyphemus' Island. Shortly thereafter, he rescues Percy and his companions, with the help of his fellow hippocampi when Blackbeard's ship, the Queen Anne's Revenge, sinks and takes them to Miami, where the hippocampi start sneezing and feeling sick because of all of the pollution. The hippocampi drop them off to swim the rest of the way as a result.
When Tyson creates a shield for Percy, one of his designs on it is of Tyson riding Rainbow towards the Princess Andromeda.
After the quest, Tyson rides him to the Cyclopes forges.
Rainbow is called near the East River when Percy is curious to know if Tyson is all right. When he learns about Tyson becoming a general, he asked if he would still give him apples to which Percy said yes. Percy asks Rainbow and his friends to carry them to Camp Half-Blood.
After Camp Half-Blood detects the arrival of Reyna, Nico and Coach Hedge, they send Tyson and Ella out to meet them. The two ride Rainbow to the yacht the group is on and rescue them from Michael Kahale. After learning of how Tyson reached the ship (and a bit of confusion when he says Rainbow brought him as Reyna thinks he means an actual rainbow), Reyna has Tyson get Rainbow to carry himself, Ella and Coach Hedge back to camp.
- Much like Percy has Blackjack, Hazel has Arion, and Jason has Tempest, Tyson has befriended an equestrian creature on a quest. Ironically, Tyson met Rainbow on the same quest Percy met Blackjack.
- Rainbow is also the only equestrian creature met so far that is not scared or afraid of Tyson because of him being a Cyclops.
- Tyson named him Rainbow because of his multi-colored skin.