If your not sure how your dog will respond to horses here's something to try. As soon as you know a horse is coming down the trail just calmly take the dog as far off the trail as you can, and observe how junior reacts. You might want to use food to focus fido's attention while the horse passes by. If your dog will not take the food there is more of a chance it could have a problem with horses. Ideally you got your canine prodigy to do a little something for the food, such as sit.
Some dogs require some kind of correction to suppress undesirable behavior around horses.
If this is the case look for a professional trainer or behaviorist you feel comfortable working with. How you handle your dog can have a big impact on how your dog responds when working under stress. This is where having an optimum relationship with your best friend can really pay off. You can increase your dogs stress tolerance by how you handle the dog during practice sessions. It also helps if you have conditioned yourself to handle the dog in a consistent manner when you, yourself are under stress.