Thanks for posting this Renee, I am very interested to know what you lovely people say as my mare did this to my other mare just this afternoon... N we are uncertain if she is pregnant or not...
Yep, hormones. When heifers here do it (all the time) the older farmers call it standing heat. I always thought that meant they'd stand for breeding, but it's said that both the standing and mounting mean the same thing.
We have pregnant mares mount others often. I have heard, though have never researched it, that a mare mounting another mare may be a sign that she has an ovarian cyst. Again, I'm not sure that's true-have never checked into it. I had a maiden mare last year that kept mounting our other mares. She is 6 years old and a maiden, though she has only been covered through 3 heat cycles. Will have her examined soon when the vet comes out for a gelding party.