That's why I suggestsd fungal or environmental. There is a pattern called machado spots or manchano spots - LOL, I can't for the life of me remeber the spelling. It's basically exactly like the "Appaloosa" looking mare first pictured in the link but is almost exclusively associated with horses in Argentina. It pops up in all breeds I believe but is completely localized. There is some discussion as to whether there's something in the water causing it (literally).
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