Nice old horn, I wouldn't just try filling the hole. A patch would look and work much better. Make a paper pattern, it will be cone shaped, that fits around the horn. Use it to cut a thin piece of rawhide that is a bit shorter than your pattern. Soak it in water until it is very flexible, wrap it around the horn and see how it fits, trim to fit, there should be a bit of a space between the ends because when you put it on the horn it is going to stretch a bit more. Stitch it together off the horn, you can butt stitch the ends but an over lap works well.
Once you have the cone made wet it again and slid it down on the horn, pull it down tight. As it dries it should tighten up but if you want you can smear a little glue on the inside before you put it one. Just make sure and wipe off the excess. If you want you can stain it before you put it on, strong Tea or Coffee works well on wet rawhide or you can stain it after it is on. Another option is to leave it as is and put Brown shoe polish on it and buff it after it dries. It should look like it is an old repair.
Hope I covered everything, if not just ask.
Tim C.