Cap horn's are very common in Appalachia. They were small horns 3" to 5" long usually with a pine plug that was sometimes held in with very small nails, a sliver of wood or maybe thorns from a honey locust or Hawthorn tree. They may have started out as a priming horn and when they changed from flintlock's to percussion, the horn was re-purposed. Throughout the years I have found several in antique stores, museum's and personal collections. They were never fancy, usually having a flare near the spout and some with a small eye screw in the butt plug.
Kind Regards
Frank Barker