Most people love dogs, but horses can easily be spooked by them. Nothing worse than encountering a lose dog with a history of aggression, chasing joggers or bicyclers, or going out hunting other animals on their own, they can easily turn on a horse. Chasing is in a dog’s instinct, just like flight is instinctive to horses. The horse's natural reaction is to balk, look, turn and flee, or react aggressively (fight or flight), which in turn may encourage the dog to pursue and chase the horse even more persistently. Staying in control over such a situation can be a true lifesaver.
The best horses are trained and prepared for dangerous and unfamiliar situations BEFORE they go out on the trails. Every trail horse should be desensitized and familiar with running and barking dogs. So any time you have the chance to practice with a GOOD dog in a safe environment don't pass up the opportunity, but be careful and aware of any reactions by either the horse or dog to prevent accidental injuries to either animal.
"Canine man" Hector Hernandez, USPS Dog Trainer teaches dog bite prevention tactics. He makes it very clear: "Some dog bite prevention training is an all positive approach from an attacking dog. In some cases, the training places too much emphasis on the welfare of the attacking dog instead of the person. Some dogs will cause great bodily harm, even death, and you must protect yourself with the necessary force. Most encounters last only seconds, it is vital to convey to the dog, within the first few seconds, that you are not weak."
Ensure the safety of your horse and yourself. After taking a moment to assess the situation, if possible and safe, distance yourself from the dog. If the dog is aggressive and approaching the horse, remain calm and keep the horse pointed toward the barking dog; most, being a bit of a coward when approached, will stop, back up, and hopefully retreat, however, this tactic is trained and must be practiced with caution! If the horse can’t escape from the threat, they very likely will attempt to defend themselves with their hooves, which may have steel shoes attached. If a dog receives a powerful kick, they could be seriously injured, or killed. Some people carry pepper spray, but it would be better to only use it AFTER dismounting if it is safe to do so, since you could accidentally spray your horse or yourself in the chaos, and the hissing sound of the can could potentially spook your horse even more.