Eye problems
The horse's eye is large and lies in a prominent and somewhat unprotected position in the skull. Given the horses propensity of flight under circumstances of fright, trauma to the eye is not uncommon. Also, given the many varied occupations of horses in some rather harsh environments, it is not unexpected that the horse incurs a moderate amount of eye disease. The eye is an extremely delicate and specialized structure, and any eye problem should be dealt with immediately to prevent any permanent damage that may impair vision and affect the horse's safety to be ridden.
If you suspect your horse has an eye problem, contact your vet immediately