Congenital stationary night blindness is a vision problem found most often in Appaloosas. It's a sex linked recessive (and thus occurs only in male horses, but can be carried by mares). There also appears to be a possible link between CSNB and the Lp gene (if homozygous).
It's called "stationary" because it doesn't get worse as the horse ages. The horse has normal vision during the day time, but extremely poor night vision. (Very rarely, the horse does develop daytime vision problems as it gets older). There's no treatment or therapy and horses with CSNB shouldn't be bred. It's a neurological problem in the retina.
They can be used normally, but should be stalled at night - some horses with night blindness will stand still all night and not eat or drink, which isn't very good for them. Some owners even provide the horse with a night light. Needless to say they shouldn't be ridden or worked in low light conditions. (Imagine your character's new horse throwing them because they tried to ride into the night and didn't realize it couldn't see).