My horse had an increased startle reflex (so much so I fell off at walk!). Low/bright sunshine/car headlights on a wet day were tricky for him so had to make allowances. He was getting on a bit so retired him as didn't think it was fair on him and i didn't ever hack on roads once diagnosed.
We were told at the vetting stage 11 years ago that our pony had a cataract developing in one eye, although can't remember which. It never got to the stage of needing treatment as he retired due to arthritis/heavy rider a couple of years ago. I do wonder if there were signs of him getting to the stage Archangel mentioned, but other issues intervened.
He did sometimes used to look hard at things (drainage gully, road marking) with one eye, and that might have been effect of different light etc, or may just have been his way of keeping me on my toes
I am following this with interest as my mare has a cataract in both eyes which I think are now getting worse although I will get them checked next time the vet comes out to see her. We have a round wall on our yard surrounding an old well that everyone uses as a mounting block with plants in the middle. This summer she wont let me lead her up to it to mount from the left side but she is fine with me using it to mount from the right side (despite me being very ungainly doing this, lol). She is fine with me mounting from the left using the block in our indoor school though. She is also getting more looky and spooky at things that never bothered her before - road markings, slim gaps in concrete paths, metal covering for hose that runs to the field water troughs that we have to step over to get out of the gate to hack out.
My mare developed them at 16. She's a very bold and honest horse who started spooking at rustic things, ie a sandbag at the side of the road, while not batting an eye at the bright roadworks next to it. Once I knew why she was doing it a gentle pat and a reassure (plus ten years of a good ridden relationship so she trusts what I tell her) meant that we haven't had any problems (although we've not jumped since!). She has gone a bit worse (she's now 20), I noticed when I hung a lickit up in her stable while she was on box rest and she walked into it and scared herself to death. I no longer put fly masks on her, I just use fly fringes, as I don't want it to be any darker for her.