Strange.... but could be something as easy as an insect bite - don't you have them blood drawin' bugs down there?
HA! What blood drawin' bugs DON'T we have down here?! Funny you should mention that because one time he did actually have one of those big ol' B52 horse flys bite him on his goober and he pulled it up inside his sheath before it could turn loose. I have never seen a horse tap dance before, but darned if he wasn't doing the closest thing to it I've ever seen, at least on the hind end. I was standing right there and he starts swinging his butt in my direction, looking at me like "DO something!". I had to laugh at him first, but I did reach up in there and get it out. It was as big as the first joint of my thumb so I know it musta hurt BAD! But it was still funny. ;o)