Probably you should post this in horsekeeping group.  You'll get more responses.

General eye care tips:

one eye means it is probably not systemic (ie related to EMS/IR).  Could be an irritant, infection, injury
keep the eye clean and dry and masked
antibiotic ointment - probably not needed unless you see signs of redness and purulent discharge
steroid ointment - I really would recommend staying away from steroids unless recommend by vet.  Steroids can make some conditions worse and in some cases cause damage.
lastly - I looked at your picture and it looks like inflammation in the lower tear duct.  You would need a vet to assess this and flush.  

My personal feeling is - I don't want a blind horse so anything that lasts longer than a few days I'd have the vet look at.

Good Luck!
--
Eva and Anske
October 2017 Corvallis OR
Case History Anske 9yo Friesian Mare https://ecir.groups.io/g/CaseHistory/files/Eva%20and%20Anske