The only thing that saves the dog is luck. Yes, that's a spoiler for the ending, but of course you want to know that the dog doesn't die, so there you go. And the better part of the book is the explorations of everyone's attitudes, perspectives, motivations. Those get pretty nuanced, complex, mostly (though some community members did def. overreact).
The book would have been better though if the situation itself were explored more carefully. I can't believe the choices are put him down or think about maybe sending him to a shelter in the city with a note in his record. Never did they even think that maybe the dog was trying to play with the horse, never did they consider any other options.
And the horse's owner, well, I'm glad she let her anger cool, but goodness. I dropped charges on a dog that actually bit me, with only the proviso that the owner fixed the fence. And I only needed a brief conversation with the owner to make that decision, to realize the dog was loved & would be kept from a repeat incident. The horse owner was practically a godmother to the boy; you'd think she'd be more compassionate, not so reactionary.