We have a mare just like this. She feels quite comfortable around other horses, but when she is in heat the others avoid her automatically, and alsu people have to be very careful around her. She squeals, lays her ear back and may kick and/or strike, but, once mounted (and if not in heat) she is an angel. If in heat, no leg pressure can be applied at all, because she acts as if the rider was a stud on her. Lots of squealing and urinating, and no forwards motion at all. She is on various medication and only a couple of people are allowed to handle her, but nothing seems to help.
I have come a long way, to surrender my shadow to the shadow of my horse.