If you're old enough, you just might remember Misty of Chincoteague, the story by Marguerite Henry about the wild horses of Assateague Island.
Here's the backstory from the National Park Service:
Assateague's wild horses are well known, even to many people who have never been to the island. The "wild" horses on Assateague are actually feral animals, meaning that they are descendants of domestic animals that have reverted to a wild state. Horses tough enough to survive the scorching heat, abundant mosquitoes, stormy weather and poor quality food found on this remote, windswept barrier island have formed a unique wild horse society. Enjoy their beauty from a distance, and you can help make sure these extraordinary wild horses will continue to thrive on Assateague Island.
Local folklore describes the Assateague horses as survivors of a shipwreck off the Virginia coast. While this dramatic tale of struggle and survival is popular, there are no records yet that confirm it. The most plausible explanation is that they are the descendants of horses that were brought to barrier islands like Assateague in the late 17th century by mainland owners to avoid fencing laws and taxation of livestock.