I think you may be referring to condition called Strabismus. Strabismus is an eye condition in which the eyes don't point in the same direction. It can involve one or both eyes. The affected eye, is the right eye in your horses case. An eye with a strabismus will show excess sclera (white of the eye), and the pupil will appear to be slanted abnormally. A horse with this condition will have difficulty seeing out of the affected eye which will result in stumbling, bumping into things on the affected side, and increased nervousness (spooking easily).
The causes for Strabismus in adult horses include: trauma to the eye or head, space occupying masses of the orbit, infection or inflammation. Infection or inflammation may have originated in the eyelid, or tissues surrounding the eye, such as the sinuses or tooth roots. The prognosis of this disease and successful treatment will depend on the cause of the eye abnormality, which must be ascertained by a veterinary ophthalmologist.
Thank you for the information. I will be contacting the local Vet. school to examine my horse.