Wild island horses may have legendary origins

Assateague's wild horses are well known, even to many people who have never been to the island.
Did the Mediterranean break through the Dardanelles to form the Black Sea when water levels rose long ago? Was a lingering folk memory of such a catastrophe behind the Noah’s flood story? Could the Thera volcanic eruption, of around 1,500BC, have caused Pharaoh’s ten plagues?
Are most fables, even modest ones, rooted in fact?