Afire mares

Fire Mares are a species of horse on the planet Krull (possibly others), their appearance is that of normal horses, except that their apparent top speed is 1,000 leagues in 24 hours. Going their top speed causes their hooves to catch fire, but causes no harm to the horse. They are wild, and roam the Grimaut Mountains. Colwyn and his comrades rangle the horses so that they can get to The Black Fortress before the sun sets (1,000 leagues from where they were.)