KDW, interesting, had to look that one up. I don't think it's vitiligo, though, as mentioned the hair is very clearly GOLD, not white. The skin underneath is still dark.
Churumbeque - nope, no rubbing, and he wasn't blanketed last winter at all. He's pastured 27/7, for 3/4 of the year, and stalled at night only in the winter. As mentioned, he has had no injuries, rubs, or anything to this area, though he has gotten a fair few bites elsewhere. I see these horses every day, many times a day. They live at home, they are not boarded. See the picture of him just coming out of his winter coat this spring in my OP, that was taken in like, March. Clearly no rub, and no gold spot then. Feed hasn't changed, routine hasn't changed, the only thing that has changed recently is the type of fly spray I am using, but the gold spot appeared before that switch was made.