I think the problem here, began because the dog HAS been allowed to chase horses in the past without being stopped. Stands to reason that he thought it ok and then it escalated. So I'd want to ask, exactly how much real training has the dog had? I'd bet little. Dogs - all dogs, should be obedience trained. If they are around other animals, then they must be trained until they are rock-solid, on recall and down/stay.
Anything else risks the life of the dog, when he is one day kicked by an unhappy horse.
Anyone not willing to train, should not own dogs.
Agree with Lizzie here! I went with my horse and dog to the beach yesterday and I do not have my dog on a leash and there comes that woman with her young dog and ask me if she can look at the horse while on the leash,
I said: Of course as long as your dog stays on the leash.
Then she asked me: How do you handle your dog off leash with a horse?
My answer: Well, my dog is trained to come when I say "come" and "heel" and "stay" and all the other things, AND I put more than a year of training (YES, with a shock collar) in it and in my horses (not with a shock collar hahahaha)...
She said: Oh, wow, that is ideal then. But who has the time to do that?
I said: It is not ideal, it is basics for taking your dog out, off leash, with or without horses... if your dog does not come when called it has no business to be off leash! And if you are not able to train your dog to listen to you and you are not willing to keep it on a leash at all times, then you should not own a dog as it is a risk for anybody around you!
So what I am saying is, the shock collar worked great for me, great training tool, and my dog needed to get shocked a few times in her life to have the best dog life a dog can have (going on trail and beach rides EVERY DAY) - I think my dog thinks it was well worth the shocks