Ophiotaurus

The Ophiotaurus (meaning Serpent Cow in Ancient Greek) is 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 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 and Blackjack. Unlike most monsters, the Ophiotaurus is actually very tame, sweet, and innocent. Thanks to Poseidon, 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 murdered by the Titan's ally during the first Titanomachy, but it's 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 murder before it could turn to the Titans side and defeat the gods. Percy first meets him trapped in a net. 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 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 it's 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.

The Last Olympian
Percy sees him in his sphere-shaped aquarium during the Battle of Manhattan. He tried to remain out of Kronos' notice, and got out of the conflict alive.