Brought my horse in tonight to find him covered in these bumps. They are itchy and a couple of them had a tiny orange-y crust on top (apologies if you've just had dinner...).
He was turned out rugged, so nettles/fly bites unlikely, and no ants' nest in or near his field. My pet theory is a trapped spider, but what does HHO think? Have daubed him in aloe gel for the night, which does seem to have helped.