A legend grew up concerning Carn's lord of the castle that he had horse's ears and the barbers who crossed the isthmus once a year to shave him were then killed to prevent them spreading the word. It sounds very like the fable of King Midas who had ass's ears, but rather more gory as eventually one of the barbers slit lord Carn's throat, and is also a familiar tale in Irish folklore, e.g. here http://www.irelandseye.com/aarticles/cu ... /myth.shtm. [notice the king's name was originally Maen i.e. stone, as in menhir]
The story seems to contain several typically Megalithic elements (Hermes is associated with barbers as well as traders, tricksters and the rest) unsurprisingly since Brittany is famous for its standing stones. Horse-y islands have come up elsewhere though; are horses or horse-eared lairds, Irish or other, a particular feature of islands and/or causeways (chausee in French)? Perhaps the idea of a hybrid is appropriate for somewhere that's not quite land or sea but why horses/asses?