Last Saturday, when I went on a (mostly) very nice trail ride with Pie, there was one odd incident - Pie caught sight of a group of people walking, about 75 yards away, and was very worried about them - I actually thought he might spin and bolt (even though we were with another, calm, horse) - he clearly was having trouble seeing them well enough to know what they were. Once they got closer, he was fine. It was a very bright, sunny day, and they were to his left - this is relevant information. Then later this week, when I was riding him in the indoor arena, he was very nervous and spooky about an area where there was high contrast between dark and light. Pie'd had some spooky moments in the past - we think mostly attributable to Lyme disease - but they'd stopped being a problem a while ago - and all of a sudden the problems - which seem to be visual - are back. Pie is not normally a spooky, nervous or reactive horse - his idea of a good time is to stand still and chill out.
I took a good look at his left eye - his spookiness is usually on the left side - and saw something that concerned me enough that I made an appointment for our vet to come out next week. We'll have them look at his right eye too, but that appears to be less of an issue.
All horses have what is called corpora nigra in their eyes - they're sort of ruffled curtains that extend from the iris and are supposed to provide extra eye protection in bright light. Some horses, however, develop cysts - round growths - either attached to the corpora nigra or free floating. Sometimes these don't cause vision problems, but sometimes they do - if they're large enough, they block all or part of the horse's vision, particularly in bright sunlight.
This site has a lot of gruesome eye photos, but if you scroll down, you'll see several corpora nigra cysts - the one in Pie's left eye is larger than any of the ones pictured.
This site also has some good information about various eye ailments, including cystic corpora nigra.
And here are a pair of photos (found on a forum site) showing a cyst in lower light and then in bright light. The larger cyst is on the front end of the eye (to the right in the pictures) - Pie's is on the back end of the eye:
Pie's pupil does not appear as obstructed as that in the photos in bright light, although his single cyst is even larger than the one pictured - I think it has gotten much larger over the last year.
Dawn has what appear to be several small cysts, but they've never caused her a problem, and Red's vet check noted that he had very large corpora nigra - but no cysts.
We'll see what the vet has to say - the good news is that, if this is the problem, treatment with laser surgery is often very effective in resolving the issue - the surgery deflates the cyst, resulting in much improved vision.