There is a mare named Annie at the barn I board and work at that is friends with another mare, Cammie. Annie and Cammie have been friends for a few years, and then one spring day I walked outside to see Annie 'winking' and backing into Cammie. At first, Cammie was annoyed and tried biting and kicking her away, but after a while Cammie eventually mounted her. When I took Cammie out and put her in another paddock and they went nuts. They kept neighing at each other from across the barn, and they paced the fence for a while until both of them calmed down. I've tried to keep them separated, they only act out when Annie is in heat not the other way around. But since I've kept Annie away from Cammie, whenever Annie is in heat she annoys other mares, and has gotten two other mares to mount her. One of the mares has even had three foals. You'd think she'd know the difference. Lol I was wondering if anyone else has had this problem before or has heard of it? Is it a hormone imbalance?
My mares do this in the spring. I thought this was weird but it's not harming anyone or anything. The only problem is they get sore fast and maybe it is a good idea to separate them so you don't have a sored horse.
I don't see anything wrong with it as long as no one gets hurt (ex. The mare being mounted kicks, or something). Especially if they have been buddies for so long. It will cause more distress if you keep them separated. I have a broodmare that bonds to whatever she is in with that moves and she will pace the fence if it is taken away from her, although if you take her away she doesn't care besides nickering a couple times. I'd put them together again, unless something changes.
April will mount the other mares when she comes into heat as well. No one has ever gotten hurt...but now that she is boarded she has her own turnout. Heifers (cattle) do it when they come into heat too...
I actually read an article in Equus a few months back. It was about a mare that would mount other mares in heat. The mounting mare appeared to be a mare, didn't have any male bits and even peed like a mare. She was a rescue case.
However, upon a vet exam, the vet realized that the mounting mare was actually a stallion and that his parts just got jumbled up while he was being developed. I do believe he was gelded.
I'll have to re-read the article so I can get more information if you'd like.
Not that that is what's going on in your case, but just thought I'd mention it.