Why Is a Horse Always Mounted from the Left Side?
The custom of mounting and dismounting a horse from the horse’s left side probably started in very olden times, when men wore heavy swords that hung down from the left leg. It was easier to throw the right leg across the horse’s back, since the long sword wouldn’t become entangled with the rider’s leg as he mounted.
Today, one of the first things a rider learns to do is to mount the horse from the horse’s left side only. Most horses become used to being mounted from the left side during their training. Someone mounting from the right side might startle the horse.
By the era of the American cowboy in the late 1800s, mounting from the left had become entrenched as the only “proper” or “civilized” way to do it. The Native American, however, being unacquainted with and unencumbered by European anachronisms, mounted from the right, and a horse trained to this became known in cowboy parlance as “Indian-broke.”
Mounting from the left may have become the rule because early horse trainers noted that horses demonstrated a preference for being approached and worked from that side, which is now backed up by modern research.
Testing reveals that the horse’s left eye responds quicker and more strongly to stimuli, and they like to keep humans in that line of sight. To reinforce this, the left side of the horse became known as the “near” side and the right as the “off” side.