One day when John and Beauty are out riding, they spot a boy trying to leap a pony over a gate. The pony keeps refusing and turning away, though, and when he does, the boy whips him. And then the boy takes it a step further, beating and kicking the poor animal (13.1).
As they watch, the pony throws the boy, who falls headfirst into a hedge; the pony takes off. John laughs and says that it serves the boy right.
The boy calls for help, but instead of offering some, John rides to farmer Bushby's nearby to tell him about the boy, who turns out to be Bushby's son, Bill. John tells Bushby that he saw the boy "whipping, and kicking, and knocking that good little pony about shamefully" (13.7), and says he left the boy rather than helping out.
The farmer says it isn't the first time Bill's treated the pony that way, and he plans to teach him a lesson. (Although we hope he doesn't teach that lesson the same way Bill tried to teach the pony…)
Back at Birtwick, John tells James about what happened. James describes Bill as a bully who caught flies and pulled their wings off. Delightful, right? James talks about a time Bill got in trouble for torturing flies, prompting a lecture from the schoolmaster about cruelty and the Devil (13.10). So according to Black Beauty, those kids in your class who torture insects are pretty much pure evil.