Epoxy is good for filling holes. It tends to show, on cow horns, but if it is colored black it is virtually invisible on buffalo horn. Stew-Mac, a luthier’s supplier (formerly Stewart-MacDonald), sells a two-part epoxy that is really thick, like a gel. They also have “powder stains” in a variety of colors (including black) that can be mixed with adhesives like epoxy to provide color. Stewart-MacDonald used to have similar powders in only two colors… ebony/black and rosewood… that they sold in little pill bottles. I bought a bottle of each 50 years ago and still have most of it left. It doesn’t take much.
That is an impressive crater, though. It may be hard to conceal completely. I have seen photos of old horns encased in a tight-fitting wrap of thin rawhide, stitched in place. That might work.
Crazy Crow is the best place I know to get raw buffalo horns for powder horns. The ones they sell are big, mature bull horns for $15.95. These go on sale for around twelve bucks once a year, but it’s hard to predict when that will be. I bought three last year, and they are all great horns. The point being that you might spend $30 for epoxy and a bottle of powder stain, not to mention your time expended, to make the old horn work, or you could buy a fresh horn for $16 that needs no repair. I understand if you have a sentimental attachment to the old horn, though. No disrespect intended.
Good luck with your project!
Notchy Bob