Not with a horse, but I've got limited sight in one eye and it makes me very jumpy. I can see enough to see things but not enough to work out what it is. Its horrible. I can imagine a horse with similar would be very spooky and worried.
Eyes are definitely a vet issue. But I have managed a horse with stationary night blindness, a horse with one eye and my beloved oldie now has reduced sight as a result of age.
And yes - sight problems are usually resolvable. You have to find a way for the horse to feel safe. Even totally blind horses can live "normal" lives with the right environment. But there can also be things in the horse's background you can't control.
He’s a complicated soul. 12 year old highland wild on a hill until eight so have put irrational behaviour down to that. Yesterday we were walking calmly home he was on a lead rope. New route when he suddenly stopped looked into the far distance and froze. We were at the top of a hill with a rather lovely huge view. He trembled with fear. I did my usual of giving him cues to earn feed which eventually got him back ‘in the room’ and we got home. It was the way he stared! Very odd.
My last horse went blind due to bilateral posterior uveitis, we didn't know he had it until he was nearly blind and put his behavioural issues down to him being a little ****. He would bronc in reaction to things he could hear but not see and was always spooky on hacks. When he eventually got to the point where his his vision was nearly gone he became very quiet to ride, I realised later he was relying on me. I wish I'd known what he was going through. He was pts while he was still topdog in the field and had a good quality of life.
I bought a young horse who was very jumpy. His breeder told me he had been picked on in the field from young, and she was very dotty herself so I thought no more of it. I got him backed with difficulty, as he bucked badly if something he wasn't expecting happened, like being touched by a stirrup. Then he just became too dangerous to get on, he would explode. And in the stable he was very unpredictable. He would jump so hard if something surprised him that he would fall over.
I tested his eyesight and he seemed to me to have no vision behind him. I called the vet and she confirmed the retinal atrophy. She said that she could refer me, but that it was extremely unlikely that anything would be able to be done. She was sure it would not be progressive.
I carried on trying to reback him very, very slowly, testing his eyes regularly. Eventually, I could almost touch his cornea before he realised that my hand was coming towards his right eye. And he was almost as bad on the left. He was a very frightened horse, dangerous to handle, so I had him PTS at four. It was heartbreaking.
I've known quite a few ex polo ponies with minimal or no sight on the mallet side. All seemed to adapt ok.
My Appy on the other hand has no obvious vision issues but the vets suspect limited sight in her left eye from her reactions. They are candidates for insidious uveitis so perhaps it's from that. She's got worse to hack and can react just as you describe. I think she can see 'something' when she's like that but her brain doesn't make sense of it so she goes into fear mode. Much better if other horses are there and they aren't reacting.
Your description of behaviour doesn't suggest a sight loss to me - horses often react to things we don't notice. To me it just sounds like he saw something that frightened him. At least he froze rather than trying to take off. And he doesn;t seem to need to take comfort in a group. But keep an eye (no pun intended!) on it. Horses with sight loss tend to become very reactive to things even we think they should have seen - like us standing near them. Changes from light to dark can make it more obvious. Some will only lead / lunge one side - either because they can see you or because they can't!
If there is any sight loss idenitfying it early gives you the best possible chance of treating it - either to improve it, to stop further loss or just to change how you manage them to give them the best quality of life. I woudn't waste a vet call out on it right now - but the next time you have one coming anyway perhaps they could check.