Hippocampi are creatures that from the waist up have the body of a horse and from the waist down, have silvery fish bodies, with glistening scales and rainbow tail-fins. They were used to draw Poseidon's chariot and seafoam was created by their movement.
Hippocampi are first seen when they arrive to take Percy Jackson, Annabeth Chase, and Tyson to the Princess Andromeda at Poseidon's request. Tyson is sad to see the large hippocampus, Rainbow, leave when they board the Princess Andromeda. Later in the book, Rainbow saves Tyson from the explosion of the CSS Birmingham and carries him to Polyphemus' Island. After the Queen Anne's Revenge sinks, endangering Grover, Annabeth, and Clarisse, Tyson suggests summoning Rainbow for help. Percy and Tyson are able to summon Rainbow who brings friends that rescue them. The hippocampi carry the group out of the Sea of Monsters and close to Miami though they won't go too close due to polluted waters. Rainbow later reappears to carry Tyson to the Cyclops forges.
They alert Percy of the Ophiotaurus trapped in a net and help him free it.
Tyson seems to get along with the hippocampi. Later, hippocampi appear when Poseidon arrives to battle Typhon. They pull Poseidon's chariot out of the river and can swim through the air as easily as they can in water.
Hippocampi are first seen caged inside an aquarium located in Atlanta, Georgia for the "Dangers of the Deep" attraction alongside nereids and sea monsters. Percy is outraged by this and tells the owners, Phorcys and Keto that they deserve to be freed. However, the two gods take him and his friend, Frank Zhang captive, but are rescued by Gleeson Hedge, their satyr protector. While fleeing, Percy promises the caged creatures that he'll be back to free them. When Leo Valdez, Hazel Levesque, and Frank meet the ichthyocentaurs, Aphros and Bythos, Leo spots a herd of hippocampi grazing in a field of seaweed. The two ichthyocentaurs promised to send one of their merheroes to free the captive hippocampi and the others from Atlanta before sending the three demigods on their way.
During a stand-off between Michael Kahale, Dakota, Leila, Reyna, Nico, and Coach Hedge, what appears to be Octavian's voice appears from below the side of the yacht and orders both sides to lay down their weapons. They do and its revealed to really be Tyson who knocks out Michael. When asked how he and Ella got to the yacht, Tyson explains that he got Rainbow to carry them though he causes some confusion about having a rainbow carry him before Nico explains he means a hippocampus. At Reyna's request, Tyson gets Rainbow to carry himself, Ella, and Coach Hedge back to camp.
- Percy says the hippocampi are "not all that smart" in The Titan's Curse.
- They refer to Percy as "My lord" because their creator was Poseidon.
- They often rely on Percy to help them. That is how he first met the Ophiotaurus (with them asking him to free it).
- A Hippocampus is also a major component of the brains of humans and other vertebrates, which is ironic since Percy said that they're not that smart.
- Hippocampi are said to be able to move through the water faster than jet-skis.
- They can be used as beasts of burden.
- Poseidon's chariot is pulled by hippocampi.
- In The Sea of Monsters, one of the hippocampi that Percy, Annabeth, and Tyson meet to get them to the Princess Andromeda is Rainbow, who rescues them after Polyphemus destroys their ride, the Queen Anne's Revenge, a pirate ship owned by Blackbeard, son of Ares, when Clarisse La Rue kept taunting Polyphemus as Percy desperately swims for the side of the Queen Anne's Revenge. Rainbow and Tyson seem to form a deep bond and Rainbow follows him into the Sea of Monsters and rescues him. He later seems to come when Tyson calls and do things Tyson asks of him.
- Tyson calls the hippocampi "Fish Ponies" because they look like a mixture of a horse and a fish.
- They are supposed to be the fully grown versions of seahorses.
- In northern European culture, there are creatures that resemble Hippocampi called Kelpies.
- The only named Hippocampus in the series is Rainbow.