Letting any dog herd horses is extremely dangerous to the dog. I have seen the aftermath first hand after a horse had had enough of being herded. This dog got stomped, broken ribs, broken leg in 3 places, smashed jaw, smashed out teeth.
I happened to be looking after my friends horses and dogs whilst she was away for the night... I took the dog to the Vet, the roof of the dogs mouth ended up being a metal plate, his leg was held together with rods, pins and screws and he ended up with no top teeth and very little bottom on the one side of his mouth.
This dog and his brother thought it was fun to herd horses, after months of this, one horse had enough and took out his anger on the one dog that was the least of the problem. The dog was so young my friend paid thousands of dollar to have him fixed up.
My advice knowing horses and having been around thm my entire life.... NEVER EVER allow your dogs to " herd " or chase horses, the dog or the horses could end up severely injured.