Flesh-Eating Horses are horses that feed on the flesh of other animals or mortals.
While Percy Jackson is at the Triple G Ranch, he agrees to clean the horses' stables to save Annabeth Chase, Grover Underwood, Tyson, and Nico di Angelo. The stables are filled with horse manure. Hercules had cleaned them using a river once before long ago. Geryon mentions that the horses prefer the stables dirty. Percy makes a deal with Geryon in which he agrees that if he managed to get the stables cleaned by sunset, he and his friends were allowed to go free.
While attempting to do this, the horses try to reach him. They are seemingly the only horses to not honor Percy as a son of Poseidon. Instead, they refer to him as "seafood" and taunt him. Presently, Percy sprays them with salt water from petrified seashells until the meat scraps are washed out of the horses' teeth and all the mess is dissolved into the ground. After Percy had cleaned the stables, the horses promise not to eat any more people, just horse food. In reply, Percy threatens to come back with more seashells and give them another bath. They agree and promise to be good flesh-eating horses from then on.
- The Flesh-Eating Horses are possibly based on the Man-Eating Mares of King Diomedes.