Delphin

Delphin is the dolphin god in the service of the sea god Poseidon. When his master was wooing Amphitrite and she fled, Delphin went in search of Amphitrite and persuaded her to agree to the marriage. For his service Poseidon placed him amongst the stars as the Constellation Delphinus. He is said to look like a normal Dolphin. He is one of Poseidon's lieutenants.

The Last Olympian
He appears briefly in the beginning of The Last Olympian during Poseidon's Battle with Oceanus, the Titan sea god; Poseidon ordered him to give orders to Palaemon and his Shark Legions.