Fire-Breathing Horse

A Fire-breathing horse is a mythical species of horse that breathes out fire through their nostrils. Percy and Annabeth find them at the Triple G Ranch and Percy meets some in a zoo in the Demigod Files.

The Battle of the Labyrinth
In The Battle of the Labyrinth, Triple G Ranch sells them for war. They aren't well cared for by Geryon and live in stables that smell horrible, because no one ever cleans out the stables. They're carnivorous and hate water. When Percy needs to free Annabeth, Grover, and Tyson, he agrees to clean out the stables of the horses in trade for his friends. Percy thinks that the horses will be his friends since he's the son of Poseidon, the creator of horses, but the horses don't really care and will eat every meat that they can find. The horses threaten to eat Percy as he gets closer to clean out the stables, so Percy ends up using seashells to clean out the stables. After agreeing to turn vegeterian and not to eat humans as long as Percy doesn't spray anymore water, Percy leaves.

The Demigod Files
In The Demigod Files, Ares's War Chariot is pulled by fire-breathing horses. They are black in color and are disguised by the Mist and a note on one of the horses saying, "Offical Zoo Vehicle". They might have manes of fire because a little girl looked at them and said, "Pony on fire!"