Old horns exhibit darkened, oxidized surfaces, both on the horn body and the wooden butt plug, and usually have a smooth, silky feel from being handled and rubbed against shirts/jackets for 100+ years. Notice how smoothly the outer edge of the horn's base has been worn down, rounding it smoothly into the edge of the butt plug wood, with the worn surface nicely darkened like the rest of the horn body... a good sign of age. The plug pins, or nails, also usually indicate age. Newer horns often have small brass nails, often with rounded heads, or small steel screws, none of which will have a darkened surface ring around the edges of the pin/nail like old horns do, from years of handling that rub rust/oxides off the nail head and onto the horn surface near the nail head. Your horn has good age on it, as seen in its darkened surface on both the horn and plug, and the dark, rusty nails attaching the plug.
Actual age on rather generic appearing horns is difficult to nail down, since different horns color, or oxidize, at different rates. But the smaller size of your horn suggests it is a percussion era horn when rifle bores were smaller, probably mid-to-late 1800s. I cannot clearly see the shape of the spout in your photos, but it kinda' looks like a typical bell-shaped spout tip with raised lower edge to catch/hold the strap retaining ring. It's not a valuable horn, but when you glue modern parts onto it, it decreases collector interest and value significantly.
Shelby Gallien