You’d likely enjoy Black Bess and the ride of Betsy Dowdy then. It’s generally not held to be fictional though.
http://www.womenhistoryblog.com/2009/05/betsy-dowdy.html
Brought tears to my eyes. And in one section, wiki tells about a foundation raising money to replenish horses lost on Assateague, an island off the coast of Virginia, that were lost in a bad storm. I guess that's where the name of the Stormy book came from.
So some of the wild horses are casualties of storms. There's another large island in the north Atlantic where there many shipwrecks and wild horses. It has been inhabitated off and on, don't know if any people remain now. I got most of my information from one of Edward Rowe Snow's account, can't remember which book or the island's name.