This poor horse whacked (a technical term meaning ‘to hit very hard’ for those of you web-browsing from overseas) its head on the lintel of the stable door two weeks ago. Does this explain what you see here?
Here is its normal eye. Note the pale optic disc on the affted side and the lack of optic nerve or retinal vasculature. There is also retinal detachment dorsal to the disc. The ischaemic optic neuropathy and detachment are both likely to be results of the trauma and are not going to improve, but the horse will adapt fairly quickly to its visual compromise on this side.