If you notice this, call a veterinarian to examine the horse. He or she can find out if there is an ulcer and how deep it is, which determines what medication is used.
Horses with eye injuries may not want their head to be touched. To perform an examination, a veterinarian usually sedates the horse or blocks the injured eye with a local anesthetic. Often a fluorescent stain is used to determine how deep the injury penetrated into the cornea.
This is especially useful for horses with conditions that take weeks to heal and do not like having their eye medicated.