What breed is Black Beauty?

In Anna Sewell's classic novel, "Black Beauty," we see the main character, Beauty, in the roles of gentlemen's riding mount, ladies' hack, fine carriage horse, and dray (work) horse. The work was published in 1877, in England. Thus, I must respectfully disagree with the response of a previous post, and state, unequivocally, that Black Beauty could not, in fact, have been an American Quarter Horse, as the Quarter Horse registry in America was not established until 1940. So, then, what breed, or combination of breeds, might he have been, instead?
The Thoroughbred registry in England was established, according to tradition, in or around 1750, so author Sewell would have been familiar with this type of horse. However, Thoroughbreds were typically used in speed events such as racing on the flat or over fences, at a distance. They were not often used as pleasure riding horses during this timeframe, and most especially would likely not have been used to pull heavy loads, as the reader finds Black Beauty doing in later chapters. In addition, the Thoroughbred has a lower head carriage, and does not waste energy with high-stepping gaits; to the contrary, Beauty is considered an optimal fine harness horse for pulling a wealthy family's carriage. He would have to have a higher-set neck and a more dynamic gait than a purebred Thoroughbred horse. Lastly, he was not only flashy in his movements, but also docile enough for a ladies' mount. In Sewell's day, all ladies rode sidesaddle, requiring a steady, dependable mount. With certain exceptions, this doesn't fit the characteristics of racing-bred Thoroughbreds, especially of the period.
I wish to posit the theory that Black Beauty was a breed known as the Yorkshire Coach Horse. With a docile temperament, elegant bearing, and yet the strength to pull loads, the Yorkshire Coach matches the characteristics portrayed in the novel. This breed was a cross between the light, speedy Thoroughbred, and the more solid, yet elegant Cleveland Bay. Black Beauty was remarked upon for his appearance and docile nature, both of which would support my theory that he was intended to be a Yorkshire Coach Horse. These horses combined the perfect blend of the flashy, upright bearing, with an agreeable nature and considerable strength. They were favored among the wealthy, as seen in the earlier years of Black Beauty's life, yet blemishes and other signs of ageing would have, in time, led to a lower value and, consequently, less-quality home and assignment. This was poor Beauty's fate, and only the intervention of a previous caregiver, who recalled fondly Beauty's innate worth, allowed for his escape from a tragic end. It is unlikely that a regular hack would have made such a strong, time-resistant impression on the fellow who saved Beauty, yet a dashing, elegant, yet kind Yorkshire Carriage Horse would have imprinted quite a memory in young Joe's mind!

This is a difficult question to answer, as nowhere in the book is it explicitly stated what exact breed Black Beauty is. We are, however, given some descriptions of his appearance, such as the fact that he is mostly black aside from white marks on his foot and forehead. In general, most readers think of Black Beauty as a thoroughbred.
Aside from the book, we can look at the film adaptations and see what breed of stallion was used to play the titular horse. In the most recent adaptation, released in 1994, Black Beauty was played by Docs Keepin Time, who was an American Quarter Horse. The American Quarter Horse excels at sprinting and is the most popular breed in the United States. While we don't know for sure if Black Beauty in the original story was an American Quarter Horse, this is some of the best information available to us.