I think the biggest difference is my mare is extremely dominant, I have had around 50-60 horses, and honestly couldn't think of a single one who would be able to take my mare on. Also, we had just gotten a new herd member for training and this really brought this behavior out. She doesn't hump or anything like that, just jumps up on. Maybe something she's picked up from the past? Anyways, from what her previous owner said, she's in no way homosexual. Before she was pregnant she wanted nothing more than a stud on her, I'm not concerned as she's only had this happen a couple times, and once I'm working with her, in no way does she act like a stud. From what I've read, it's only a combination between her dominance and being pregnant. I'll update if anything changes.