Fire-breathing horses are a type of magical species of horses that breathes out fire through their nostrils. Percy Jackson and Annabeth Chase find them at the Triple G Ranch and Percy sees some, pulling Ares' War Chariot, at Staten Island Zoo in The Demigod Files.
Ares' War Chariot is pulled by fire-breathing horses. They are black in color and are disguised by the Mist. While at the zoo, they had a note on one of the horses saying, "Official Zoo Vehicle." They might have manes of fire because a little girl looked at them and said, "Pony on fire!"