When shooting untethored horses, you sometimes need to think fast. Here, a dark horse moved behind a very relaxed horse. A relatively long lens focal length has a relatively narrow angle of view which means a relatively small area of the background is in the frame. In this case, a horse is large enough to mostly fill the frame, giving me a black background without taking any gear to the corral - or the horse to the studio.
How do I know the horse is relaxed? The bottom lip is hanging low and is slightly open. Perhaps we need to learn from the horse - but probably should not use this practice when in public.