Think portrait aspect.
The sides of this photo are not really contributing anything to the actual message or content of the photo itself; the right side is utterly devoid of any interest and the left only has the dark (mostly black) body showing. If you'd gone portrait aspect and also given the muzzle and hay a little more room at the base (without clipping the ears) you would have a much stronger profile of the horses head.
Now if you want to keep in landscape you've got to introduce something else to balance or focus the photo - have the horse looking off into the empty space to give a sense of motion and a direction of gaze; have more surroundings or something that the horse is interacting with (eg rider) in the scene etc.....