Eugh horrible. Brings back memories of a similar thing that happened to me when I lived in Portugal. I was out on a 4 year old mare who was in season. I hacked past the local gypsy horses as I did every day, giving them a wide berth as usual, but the stallion had somehow broken free from his tether and accosted us and mounted the mare from behind. She was only small and I didn't have a whip. After a lot of rearing and kicking and spinning, during which my Border terrier got knocked into the river (he came home safely later thank goodness!), we ended up going flat out all the way home with the stallion in pursuit, he would NOT leave us alone, I literally had to throw myself off as we got through the yard gate and only just managed to slam it in his face. The worst thing was that I could have easily been riding there with a group of Pony Club kids with me, I can only imagine what would have happened.