Oh nice! Love the lighting you've chosen. If you were willing to redo parts of it hoenstly I think it would look amazing if the horse was reacting in some way. Those old renaissance paintings where a horse is rearing against an opponent is very awe inspiring and could help with both your composition and gravity of the situation. A good example is this one:
Henri Regnault's Automedon
Notice how the horses are visibly distressed, showing a lot of emotion in their body language and faces!