The horse is used as a symbol in your dream. It is black, not white, and typically we use the color black to symbolize evil. Think of the evil black widow spiders, or black clouds which warn us of oncoming rainstorms. There are many other things which are black which are used to symbolize danger, or evil. The black horse was on your neighbor's side of the fence, but he managed to chase you right into your own yard and your own garage, when the whole scene turns to black. Look to your current Life right now. Someone chasing you, with the power of darkness, or evil. (Forget the horse, as it was just used as a symbol.) Who does he represent to you in real Life? Think through the current happenings in your Life. Anything happening to you which is bothersome, or causes you fear? Dreams manage to conceal the true message of the dream, as we would all wake up in shock if we found out immediately. But gently, softly you may discover who, or what this darkness represents.