It's never a good idea to think you'll be fine with an aggressive horse. If the op is on the trail and runs across a dog, just because it's under saddle doesn't mean it won't do anything wrong. That kind of thinking could get the dog, horse, rider, and dog owner seriously injured. My suggesting has already been stated about trying it in a controlled environment and air on the side of caution. A full grown horse is hard to control when it's got an idea in it'd head and decides not to listen to you especially when you're not confined in a fenced area. I just wanted to say don't assume it will be fine. Some horses are but others are not and I don't suggest anyone ever think they will be fine when it has been stated a horse attacks aggressively.
Show me a horseman who hasn't fallen and I'll show you a man who has never truly ridden.