Join Date: Nov 2012
Location: Saskatchewan Canada
If I were you, I would not ride in the same arena as she is in- if she is loose. Get your horse and go somewhere else. If she comes after you, scare her away. If she is not in your space or around you, ignore her. If you have permission, maybe you could take her in a round pen and show her you are the boss. If you don't have permission, ask to see if someone else can. Desensitize your mare so she is not afraid of the whip or rope and use that as your defence to the other mare.
All I can tell you is stay safe and try to stay out of the pen she is in when you are working with your own horse.