Re: I need help
I am far from an expert. I have a horse and work around horses every day. I also do herding with one of my dogs.
Horses don't really like being herded. In herding trials you can see sheep, goats, cows, ducks being used. But you won't ever see horses. They just don't respond the way a herder needs them to respond.
You could go to a herding trainer and learn how to work your dog. But that won't really help your situation, since horses don't herd the way sheep, etc. do and your dog will still be at risk of getting nailed by a horse. If you are really keen on guiding his behavior though, within the context of herding, find a trainer to work with. There are so many things you need to be aware of, getting expert help is worth the effort.
I would suggest that you teach a rock solid recall and never let your dog have access to the horses. Or have him on a long line so you can reel him in if he blows through your recall.