An important part of taking good horse photos involves anticipating where the horse is about to be, instead of trying to get the photo in the exact moment it happens. (This is particularly true of some digital cameras, which may have a perceptible delay between you pushing the shutter button and the camera actually taking the photo.) One of the reasons that experienced horse people make better equine photographers than people who aren't as familiar with horses is because horse people can usually accurately anticipate a horse's behavior. As Pat Parelli would put it, they "know what happens before what happens happens."
( Here, allow me to illustrate with illustrations. )
( Here, allow me to illustrate with illustrations. )