It's not about the horse

What this title is saying is that the behavior your seeing is likely not because of the actual thing your dog is reacting to. Why is your dog barking at other dogs on the leash? More times then not, when we ask this question we hear it’s because they want to meet the other dog, had some negative experience with dogs, or some other reason. Typically, we then ask how does the walk look? Usually, we get the idea that the dog is pulling out in front, doing what the dog wants when they want, and is generally not tuned into the handler. The symptom of a dog leading out in front without a care in the world with the handler is usually what I listed above. 

The first course of action would be to get a good walk started. A good walk looks like this. A dog walking at your side or behind you and is in a calm, relaxed state of mind (ears back, fore head relaxed). When they are walking like this and are concentrating on their position their ability to react is cut down. This also allows you to see the little moments before they become big moments. So get the basics down, teach your dog what to do, teach them what not to do and you will see changes.