Ophiotaurus

The Ophiotaurus (meaning Serpent Cow) was an ancient monster that could bring down the gods if it was slain and its entrails were burned. It was a half-cow, half-serpent creature. It was nicknamed Bessie by Percy Jackson, who initially assumed it was female. The Ophiotaurus is currently safe in an aquarium on Olympus built by Hephaestus, the god of blacksmiths.

History
In the ancient myth, it is said that the creature emerged from Chaos with Gaea and Ouranos. It was slain by the Titans' ally during the first Titanomachy, but its entrails were retrieved by Zeus' eagle before they were burned.

After that, the gods kept it in heavens in the combined constellation of Taurus and Cetus.

The Titan's Curse
The Ophiotaurus was the creature that Artemis attempted to kill before it could turn to the Titans' side and be sacrificed to the gods. Percy first meets him trapped in a net and nicknames him Bessie (at the time he was unaware that he was actually a male). Thanks to the Hippocampi and Percy the monster was freed. The Ophiotaurus was the center of the General's plan to bring down the gods by having Thalia Grace call the beast to her, kill it, then sacrifice its entrails as a tribute. However it was Percy, not Thalia that the Ophiotaurus follows around. This is due to Percy's rescue, and the monster's belief that Percy is its protector. 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, after Zeus refuses to let Poseidon harbor him under the sea as a "bargaining chip."

The Last Olympian
Percy sees him in his aquarium during the Battle of Manhattan when he goes into the Hall of the Gods. He tried to remain out of Kronos' notice, and got out of the conflict alive.