Hippocampus

Hippocampi are creatures that are a combination of horses and fish that are under the rule of the sea god Poseidon.

Description
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 sea foam was created by their movement.

The Sea of Monsters
Hippocampi are first seen when they arrive to take Percy Jackson, Annabeth Chase, and Tyson to the Princess Andromeda. Tyson is sad to see the hippocampi go and nicknames the one who took him to the ship Rainbow. Later in the book, Rainbow saves Tyson from the explosion of the CSS Birmingham.

The Titan's Curse
They alert Percy of the Ophiotaurus trapped in a net and help him free it.

The Last Olympian
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.

They help Percy, Annabeth, and Nico get to Camp Half-Blood after the Battle of Manhattan, due to Rachel taking Blackjack.

The Mark of Athena
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.

Trivia

 * Percy says the hippocampi are "not all that smart."
 * 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).
 * The word 'hippocampus' is a part of the brain, which is ironic since Percy said that they are not 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.
 * 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 sea horses.
 * In northern European culture, there are creatures that resemble Hippocampi called Kelpies.