There is a principle in portrait photography that says that the subject should be the center of attention. So how do you photograph a portrait with a horse? It's not easy. But I'll let you in on a secret: there is a psychological principle that says that the eye will go to the brightest part of a photo first, and then wander to other places. So, to be sure that you looked at Gabrielle first, I made her the brightest part of the photo by asking her to wear white.
I used a Canon Speedlight on a stand shooting through a white umbrella to fill shadows in this photo. I also took about 40 photos to get 1 decent one - the horse was not exactly compliant.