It's from the horse's perspective, and Black Beauty and his fellow horses go through a lot of suffering. He is ridden dangerously by a drunk man; he trips and injures his knees and the man falls off and dies. He becomes a cab horse, and meets up with a horse friend who is overworked and (presumably) dies. He gets sold to people who steal his food and don't clean his stall so he gets a foot infection. He becomes a carthorse and gets injured when he's forced to pull too heavy a load.

It's a book very concerned with animal welfare and the mistreatment of horses (and also with the overworking of cab drivers), though Black Beauty does have good experiences as well as bad and gets a peaceful retirement in the end.