from the back cover:
When Black Beauty is trained to carry a rider on his back, or to pull a carriage behind him, he finds it hard at first. But he is lucky - his first home is a good one, where his owners are kind people, who would never be cruel to a horse.

But in the nineteenth century many people were cruel to their horses, whipping them and beating them, and using them like machines until they dropped dead. Black Beauty soon finds this out, and as he describes his life, he has many terrible stories to tell.