Talk to the neighbor and perhaps offer to reimburse them for the dog and then keep him and immediately have him neutered and see about finding a way to prevent him from getting to the horses. Then I would immediately spay the females and just get it over with. If the neighbor gets crappy then I would point out that the warden will be called and then they won't get any money at all for the dog and will probably have to pay a fine. Hit them where it hurts the most and for most people it's the pocketbook. Like I've said over and over...I HATE the human race sometimes. Let us know what happens.