You nailed it!
Simon loves to share the horse grain, and our filly spit out the Ivermectin onto her food. Simon must have gotten enough to affect his vision. The timing certainly matched. We went to Northwest eye specialist yesterday at least armed with a possible explanation and she agreed. He was already getting some of his vision back in his left eye while we were there and she expects him to fully recover over the next few day.
Today he is seeing well enough to get around the house and go with me up to the barn to help feed.
Thank you for the heads up. We are all doing so much better.