Hi Tm
It's a disease. With no known cure. LOL
Interesting horn. I have no idea as to it's origin. At the moment, Stu's guess is probably as good as any.
The horn appears to have been scraped, but never polished. Probably no facilities to do so. Notice the crude, hand-made nails. The entire horn was probably made with a bare minimum of hand tools.
The rear flange, on the large end of the horn, with the two holes to attach a cord/strap: That type of rear attachement is what today's contemporary horn makers call a mid-18th Century style as made in Colonial America. Curious it turns up on this horn.
If you get a chance, get one of those long medical Q-Tips and run it around the inside of the horn to see if there is any traces of black powder.
Rick