The book treats most of the wild ponies from Assateague as a single character, starting with their legendary beginnings as marooned survivors of a Spanish shipwreck in the distant past. This story of their origins establishes the theme of The Natural World Versus the Human-Made World: The ponies’ unexpected survival and adaptation to the tiny island show the horses to be wild at heart, even if it takes them a few generations to completely thrive on their new island home.

When the story shifts to the 20th-century Pony Penning Day, the ponies’ character changes slightly. While on Assateague, they live the same life that their ancestors have for many years, but when men round them up, they become frightened, docile creatures with their heads held low and their tails between their legs. They move as a single herd and do not need fences to keep them together; they are able to be kept on Tom’s Point just because there is fresh grass there. Their behavior contrasts with that of the Phantom, who is determined to keep her freedom no matter what it takes.