Geathian horse

A Geathian horse was a horse found in and around the city-state of Geath on an unnamed planet. Rory Williams was surprised by their very existence, because he didn't consider that there were horses on worlds other than Earth. The relationship between Geathian horses and Earth horses, if any, was not known. (NSA: The King's Dragon)

Behind the scenes
Although the name is a demonym, the demonym is actually used in the text of Dragon. People from Geath are Geathians, so horses from there are plausibly Geathian as well.