The Eye

The horse has the largest eye of all land mammals. Eye size in mammals is significantly correlated to maximum running speed as well as to body size, in accordance with Leuckart's law; animals capable of fast locomotion require large eyes. The eye of the horse is set to the side of its skull, consistent with that of a prey animal. The horse has a wide field of monocular vision, as well as good visual acuity. Horses have two-color, or dichromatic vision, which is somewhat like red-green color blindness in humans. Because the horse's vision is closely tied to behavior, the horse's visual abilities are often taken into account when handling and training the animal.

Source: en.wikipedia.org/wiki/Equine_anatomy#The_eye