Our eyes are drawn to areas of strong contrast in an image. In this case, our eyes are drawn to the horse's eye. Most photographic rules can be regularly broken, but one rule I seldom break is that the subject's eye must be in focus.
Notice the shallow depth of field in this image. Even though an f/8 aperture was used, the 200mm focal length used at this relatively short distance leaves only a thin slice of this image in focus.