There is a herd of semi-feral horses on the island of Assateague, off the coast of Virginia and Maryland. Once a year, on Pony Penning Day, the islanders of neighboring Chincoteague drive the horses across the channel. They auction off the foals and some of the adults, after which the remainder of the herd is released back to Assateague.
Misty of Chincoteague was written by Horse Book Person Marguerite Henry, who was already known for Justin Morgan had a Horse and went on to write King of the Wind, Brighty of the Grand Canyon, and more Chincoteague books. She lived in Wisconsin, but she visited Chincoteague to prep for this book.
This book is for a fairly young audience, both in terms of the writing and the story. Paul and Maureen are desperate to buy the legendary Phantom, a mare who has eluded capture the past two years. They save up money, and Paul gets to ride with the salt-water cowboys to bring the horses across. But Phantom has a foal! They must buy both! But someone else buys the foal! But they get the foal back!
Now they have both Phantom and Misty. Misty takes to human society easily, but Phantom is half-wild and only learns to be ridden with very slow careful training. Then, this being a horse book, there is a HORSE RACE! Where the wild proud horse that could never be ridden is ridden by the child who has no horseracing experience, and WINS! Because, as the children knew all along, this is the special-est horse ever.
Then Phantom returns to Assateague, where her wild heart belongs. The children lose Phantom, but get to keep Misty.
It should be noted that the gender politics are just terrible, even for 1947. Paul does everything, from rounding up the Phantom and getting her and Misty across the channel safely, to riding her in the race (they pull a wishbone for it, but you know Maureen was never going to win), to making the decision to release Phantom without consulting Maureen. Throughout, Maureen is his meek and devoted helper.
Maguerite Henry also gets her facts wrong about wild horses — not just exaggerating but romanticizing the idea of a harem controlled by a dominant stallion. Wild horses (really, feral horses), live in a herd that is broken up into bands, each band consisting of a dominant mare, several other mares, and their young. There is usually a stallion hanging out with them, though sometimes two or three or more. The stallion only stays with the band for a year or two before moving on.
Another interesting point to note is that the island of Assagteague is fenced right across at the Virginia/Maryland border. On the Virginia side the horses are only semi-feral, getting rounded up once a year so it’s hardly a surprise, and even getting regular vet checkups. On the Maryland side, they are left wild, the only interference being horsie contraception so the population doesn’t get out of control. (Although they do wander around the campgrounds, so maybe not all that wild.)