Picking out feet yesterday morning, I found a huge hole in the back of his left fore, just above the heel bulb.
Now, I'm 99.9% certain it wasn't there 24 hours earlier - I'd spent a day and a half worrying about that leg, sploshing water on it, cold booting it and bandaging it and I'm pretty sure that I'd have spotted a 1cm hole with skin sticking out looking like a tiny, tiny alien had chest-bursted out of it.
It looks for all the world as if an abscess has blown, but he wasn't lame enough and it's a little bit too high to have come out of the coronet band, so I suppose he must have caught it on something somewhere. Anyway, it's being cleaned twice a day and having black powder puffed on it and it seems to be healing up nicely. Definitely no walking out today though.