It's rust.I've seen many like that, some that looked like lace dresses, with multiple tiny holes. The owner of one of these exhibited the perforated horn with no alterations, as it actually looked good for him.
Another possibility is the Ice man got mad and used his ice pick on it. It is hard to see someone shooting it with buck shot with a lot more dents on it though. Ice pick combined with rust. Could have been a experimental sound dissipater too that failed when the ice man came. Tom