Originally Posted by Percheron chick
I can guarantee you that no single dog attacked the horse. A pack of dogs or mountain lion slim chance, another horse or human more likely. A horse will barely notice a dog or coyote cutting through the pasture because they are not a threat. Doesn't sound like they know horses or spend anytime observing them.
The chicken massacre sounds more like 2 or more dogs. What ever it was will return. Keep your dog locked up so he doesn't get caught up in the mess.
It definitely isn't impossible for a single dog to injure a horse. I have a friend who had a pair of Clydesdales panic rather than defend themselves, and sustain serious injuries (mainly to their legs) from a single Rottweiler, until his spunky little Morgan/Wesh pony mix came to the rescue. The pony was able to push it back from the big horses enough for the owner to take a safe shot at it. A neighbor had her own Jack Russel damage the muzzle of her quarter horse, which was further injured panicking and crashing into the fence at a dead run. That incident happened and was over very quickly, but the horse never fought back against that dog either. A horse is generally quite capable of handling a dog, but it isn't always their first instinct.
I think the most thing for the OP to do is make sure his dog is not running loose again. It is the best way to protect himself and his dog. Even if you would stake your life on your dog having zero prey drive, the neighbor and their animals don't know that and shouldn't be stressed out by your animals visiting. Fortunately, it would be awfully hard for them to prove in court it was your dog, but it does help their case if they can prove it repeatedly was coming onto their property. Hopefully your dog is innocent and the real culprit is found out.