"Moon blindness" is the most common cause of vision loss in horses. It was named because people once thought the horse's vision came and went with the phases of the moon.
The technical term is "equine recurrent uveitis." It's most often caused by a bacterial infection that then causes the immune system to keep flaring back up. The barrier between the eye and the bloodstream becomes compromised causing inflammation of the eyes, especially around the iris. In some cases the bacteria stays in the eye - and antibiotic eye drops may be used.
Moon blindness is normally diagnosed if a horse has had two or more episodes of inflammation. Stress can make the animal worse. There's no magic treatment and it's not uncommon for horses with moon blindness to lose their vision in one or (much worse) both eyes.