My photographer was fantastic at suggesting the poses for my senior pics with my horse (and the pics without the horse, too!) He recommended a location of just beautiful green grass & trees, so there would be nothing busy in the background to detract from the overall look.
I chose to wear "street clothes" rather than anything equestrian, because it best represented me at the time - shorts, white blouse, dressy sandals. My mostly-white Paint wore her show halter and was groomed as if we were at a show. We didn't do any riding shots, as the focus of these pics is supposed to be the Senior, not the horse.
Several shots have me just standing by my horse or loving on her. My favorite shot was me sitting on the ground looking up at my horse, while she was reaching her head down to be close to me.
Have fun with it!