legendary creature, part bull and part serpent, in Greek mythology

In Greek mythology, the Ophiotaurus was a creature that was part bull and part serpent. Ovid's poem, Fasti (3.793 ff) is the only work where it is mentioned. There, it is said that when the creature's entrails were burnt, the person to do so could defeat the gods.

The Ophiotaurus appears in the third book in the "Percy Jackson & the Olympians" series, The Titan's Curse. Percy Jackson first thinks it is a female and names it "Bessie". It reappears in Battle of the Labyrinth and The Last Olympian.