In Wild About Horses, a nonfiction book which examines the history of humans with horses, author Lawrence Scanlan devotes one chapter to ponies, and attempts to understand the phrase "pony character," as used by the breeders and owners he interviews. While he maintains he never quite got the whole idea, he came up with a good starting point:
I imagine that a pony with pony character has a strong sense of his own self.
Although Wesley Dennis did many pictures of horses, there's something infinitely appealing about his ponies. Their shaggy roundness offsets their ever-alert ears, making a portrait of scruffy, everyday realism that is so much more inviting than the elegant perfection of, say, Anderson's flawlessly long-striding Thoroughbreds or Savitt's athletic hunters. And maybe that is another part of pony character; reality. You never hear about a pony having "the look of eagles;" there are no legends of wild ponies leaping to their deaths to escape the mustangers. They're not fanciful or legendary. And Dennis's ponies look like realists.
Dennis's collaborations with Marguerite Henry were the most famous, but he illustrated a slew of other pony books where that 'pony character' came through the illustrations.
The Wesley Dennis website