That's a neat little horn.
It might have been a cap horn. Old-time percussion caps were susceptible to damage from wetting, and a good horn will be airtight. Also, if the lid of a cap tin gets a little rusty and sticks, and you pull with too much force, you can spill caps. This is less likely to happen if the caps are stored in a horn like that. During the percussion era, people used a lot of ingenuity in designing cap-holders, often affixed to the strap of the shot pouch, which would keep a small number of caps readily accessible. The cap horn would be used more for storage or protection of a larger quantity.
My dad (born in 1907) grew up shooting muzzleloaders. He liked carrying his caps in a cap horn instead of a tin, and he made several very similar to that one.
Thanks for showing that cool little accessory!
Notchy Bob