A Geathian horse was a horse found in and around the city-state of Geath on a certain planet. Rory Williams was surprised by their very existence, because he didn't consider that there were horses on worlds other than Earth. (PROSE: The King's Dragon)
Behind the scenes Edit
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.