What exactly is she doing? Squealing, kicking, etc? Usually when strange horses are put in together they go through an adjustment period. As long as they have plenty of room to get out of the way and away from each other, they adjust. I have known some horses that are rather asocial though-an appy mare we had would suddenly decide the horses across the pasture were too close and start backing toward them, kicking out with her hindlegs every couple steps, the witch!! Sigh..... Maybe your girl just needs time.