The answer to this question can be found in paragraph 6 of chapter 1. Black Beauty is told from the first-person narrative perspective, and readers are likely to assume that the narrator is human; however, a few details in the early paragraphs cause us to quickly re-evaluate that assumption. Beauty tells us that he survived on his mother's milk before being able to eat grass.
Beauty also tells readers that he wasn't the only colt in the meadow. There were six other colts that he would run around and play with. Beauty says that it was great fun to run with them as well as play rough. On one particularly rambunctious day, Beauty's mother calls him over and explains to him that he isn't like the other colts. They are good horses, but they are "cart-horse colts." They are rough and haven't learned their manners. Beauty, on the other hand, is "well-bred and well-borne." Beauty has been bred to a high standard, and there are high expectations of him as a horse. Beauty's father is a well-known horse. His grandmother was well known for being exceptionally gentle, and his grandfather is a prize-winning racer. Because of all this breed heritage, Beauty's mother expects him to hold himself to a high standard and behave in a way that reflects his family's good standing. This means no biting or kicking in any kind of play.