It seems to me to be genuine horn (not a plastic one) but it looks to me that the piece may be a scout project or built by a reenactor. 

The horn is about 8.5" short way from point to point (almost 11" following the bend); about 2" x 1.5" oval at the large end. The large end has a wooden carved plug that is attached by 5 small nails and 2 flathead screws, all of which have patina. The plug has a knob around which is tied a leather shoelace. The other end of the shoelace is threaded through a hole in the small tip or the horn, which attaches to the larger part of the horn with a dowel. It doesn't seem to me to be a very secure fit, and would likely come unattached if one was carrying it.  The photos show it all, I think.  I'd be happy to answer any questions.