When my sister's paint was in the herd, she was the alpha. She was pretty a flighty, herd bound mare, and very reactive. It seemed like the herd was constantly on edge when she was the alpha.
When we sold her, Willow took over. She's a pretty witchy and independent mare. The other horses get out of her way when she comes through. She'll chase the other horses off if they get to close, but doesn't go out of her way to bother them. She is also extremely protective of the herd. If the coyotes come around then she is the 1st one out there chasing them off.
I was worried about putting the yearlings in with them because of her. She actually looks out for them. The babies go to her when the other horses are bothering them.
All and all the herd is a lot calmer with her in charge.