Ophiotaurus

The Ophiotaurus is an ancient monster that could bring down the gods if it was slain and its entrails were sacrificed. It was a half-cow half-serpent creature. It was nicknamed Bessie by Percy who initially assumed it was female, although Grover later informed him that it was a male. Percy saved it from a trawler's net after being fetched by the Hippocampi. Unlike most monsters, the Ophiotaurus is actually very tame, sweet and innocent. Thanks to Poseidon, the Ophiotaurus is currently safe in an aquarium at Olympus.

The Titan's Curse
In The Titan's Curse, Percy first meets him trapped in an net. Thanks to the Hippocampi the monster was freed. The Ophiotaurus was the center of the General's plan to bring down the Gods by having Thalia call the beast to her, kill it, then sacrifice its remains as a tribute. However it was Percy not Thalia that the Ophiotaurus follows around. Grover later takes it to Olympus, and Percy sacrifices the Nemean lion's pelt to ensure they arrive safely. Deciding to allow it to live after a heart filled testimony by Percy, Poseidon and Hephaestus made a sphere-shaped aquarium for it on Mount Olympus.

The Last Olympian
In The Last Olympian Percy sees him in his aquarium during The Battle of Manhattan. He tried to remain out of the Titan's notice, and got out of the conflict alive.