The specific laws and penalties for scaring horses or livestock with your car vary from jurisdiction to jurisdiction. However, in general, you can expect to be fined if you are caught doing this. In some cases, you may even be arrested.
If you see a horse or other livestock on the side of the road, it is important to slow down and proceed with caution. You should also avoid making any sudden movements or loud noises that could spook the animal. If the animal does become spooked, pull over to the side of the road and allow it to pass.