Pampas grass is not toxic to horses.  It matures very tall and course, so if a horse were to eat it, the rough edges could injure the horse's lips and mouth.  However, if horses are presented with more palatable forage grasses, they would likely choose the forage grasses over the pampas.  

Before you plant it, you should know that it grows aggressively and is considered an invasive plant.  It is possible that it could spread into the pasture area and outcompete the desirable forage grasses.  It is very difficult to control a "weed" grass in a stand of desirable grass species.  You may want to consider some non-toxic hedges rather than a grass for a windbreak.