Summary
Your horse's eye looks collapsed and is draining a jelly-like fluid. There may be swelling of the eyelids and the eye may be difficult to see. In most cases, horses are in severe pain and will hold the eye closed, making any assessment difficult or impossible without veterinary help.
If you can see it, you will likely see an eyeball that is smaller than the normal size, with or without an obvious tear in the clear surface. The iris may be pushed through this - a small, dark mass bulging from the surface of the eye. A clear liquid may be running down the face.
Eyeball rupture is rare and usually the result of a severe blow or direct trauma to the head. The most common cause is a when a horse falls to the ground and sustains an impact injury when the eye hits the ground directly. Chronic, long-standing eye infection of the cornea (corneal ulcer) can also ultimately result in perforation leakage of the inner liquid (aqueous humor), and collapse of the eyeball. A long standing ruptured eye becomes shrunken, raisin-like.