Now the real problem starts....she has been very aggressive w/ our other horses the past 2 weeks. On trail rides she will just reach forward aggressively, and what seems out of the blue, and bite the horse in front. She has even spun around and started to kick the horse in front of her after the bite....it always seems to be the horse in front of her she directs her anger towards. At first she seemed to single out other mares, but yesterday it was directed at our percheron gelding...mr. Calm and steady. He just stood there while she nailed him in the rear. Unfortunately the 3 times she's done this, we have had riders on her that didn't do anything except hang on for the ride! A week ago we were thinking "hormones" and tried to use her strategically placing her away from other mares and looking for something to give her during that time to help her out. Now that it's been 2+ weeks, we're not sure if her heat cycle is the problem. Any ideas?