Misty of Chincoteague by Marguerite Henry, 1947

This horse-story tells the true tale of Paul and Maureen Beebe who plan to buy a pony. In the annual pony penning, Paul corrals Misty, a new foal, as well as her mother, Phantom, a mare that has escaped penning for the previous two years. Paul and Maureen both instantly love Misty and want to buy both Phantom and Misty, but the fire chief instead sells them to someone else. However, the buyer ends up winning a different pony, so the Beebe children get to buy them after all. They gentle the ponies and train them, but eventually they allow Phantom to go back with Pied Piper, the stallion of her herd. While the story is fine, it's difficult to get past the annoying dialect, which turns these characters into complete hicks.