There are a lot of factors that could cause bad behavior in your horse, and without knowing a lot more about your horse's environment and background, we cannot resolve this situation. However, you should contact the horse's previous owner, or people who were familiar with the horse's behavior in its previous home, to see if this aggressive behavior is new or a continuation of previous behavior. Then the horse should be examined by a veterinarian to make sure the behavior is not caused by a physical problem. If the bad behavior is something that you decide that you can try to solve, you should find a horse trainer with a good reputation and get him/her to work with you and the horse to solve this problem. The answer may be as simple as reminding the horse that it is submissive to people or changing its environment, or it may be something that requires more intensive training. Either way, you should gather any available information about the horse and quickly make a decision about how to deal with the horse so that you do not get hurt.