The thin area looks like normal worm damage to me. When horns are put out of sight for long periods and not protected from worm/larva damage [moth balls], at times they get eaten like this one. You can see the worm holes in surface in the thin area, and much of the horn has been eaten away underneath the thin area. Horn is really hair that has grown together, and moth larva [same ones that eat wool clothes] love to eat it. It may eventually break through if handled too much. I've heard of experts using epoxy to fill worm holes and strengthen thin/weak areas on valuable horns, but I've never seen it done, and I'm not sure your horn would justify the expense. It's a decent horn to enjoy as-is, just don't drop or bump it too hard!
Shelby Gallien