A horse can see two things at once, one from each eye. That allows each side of its brain to work separately. Like humans, horses have a dominant side (right-handed or left-handed); however, unlike humans, horses need to be taught things twice: on the right side and on the left side. The expression in a horse’s eye is often thought to be a good indicator of their behavior, e.g., wide open with white showing (and not an Appaloosa), scared; half closed, sleepy, etc.

A horse’s hearing is much keener than ours, and their eyeball is the largest orb found in any land mammal, and has a correspondingly oversized retina.   https://esc.rutgers.edu/fact_sheet

It’s been observed that which eye a horse looks at you with reflects his earlier experience with you–whether positive or distressing.

 I’m afraid I’m not a horse person, just found this interesting.  Just think what the disqualified Kentucky Derby horse must have been sensing, seeing the many horses either side of him at the same time.

Image: https://www.pexels.com/search/horse%20head/