I just went to the barn and separated Kate safely from the other two. I tried to just pull Sally (bitchy horse) out of the picture but she went psycho. I'll do that part this weekend when I can be there to make sure everyone stays safe.
That bit reminds me of my mom's boss mare. Goes nuts when separated from her herd, has anxiety problems (paces the fence into a total sweat). My mom teases that her boss mare gets more of a workout being left in the pasture than she would on a ride. Even injured herself trying to bust out of a stall at the vet's last year when my mom left her overnight.
You may need to keep her busy and give her lots of attention and reassurances. She may feel threatened by this newcomer, and doesn't want her buddy snatched up by the away from her.
You could always try scolding the cranky horse when she turns aggressive, like charge at her when she takes a lunge at the addition. Let her know that she is behaving poorly, and you don't approve. I do this with my mom's herd boss, she learned that I don't tolerate idiocy while I am in the pasture. I have never had to more than mock charge the aggressor, and if I am carrying a lead rope, I will admit to swinging that above my head for added drama. Point of it is to force the aggressor to "run" from the herd, which is not what they want to do. She knows she is top dog, and so do the other horses, she doesn't have to prove it while I am there.
Wish you luck. And remember that you know your horses best, and are free to choose the most appropriate solution that works best for you and your horses, and hope you can get them to get along with one another :) Posted via Mobile Device