My dog is a GSD/Dobie mix and he has a high herding instinct. Where I had my horse for a summer, there were lamas. My dog rounded them up to a corner. I didn't teach to ask him to do this. It was pretty cool. Until later that afternoon when my horse and I were out for a hack and my dog came and nipped her on the hocks. And was barking to get her to go where he wanted. I quickly corrected the behavior and I put him through a little dog manners course I set up.
I think that the most important thing to remeber when picking a dog is that they have horse sense; not neccesarily what breed they are!
I know my German Shepherds often loves my horse. I've often found her racing my horse from the other side of the fence, although I don't take her out with me on rides, i've found that with my horse, she is like she is with her best friend, they go every where together, swimming, trail riding, even just sitting in the shade on a hot day.