Had this happen many years ago the mice chew the rawhide so took it to a leather repair shop that was a friend of mine and he took off the rawhide and replaced it with leather. He rebuilds saddles so he knew what he was doing.
Posted 2014-09-27 1:22 AM Subject: RE: Saddle horn fixes
Expert
Posts: 1482 Location: on my horse
Either take it somewhere and get it replaced with leather, or go with tape. You can get duct tape in 100000 colors now and some of them aren't even totally redneck I just taped up my horn (same problem) and part of a stirrup with duct tape because I can't afford to get it fixed right now (don't worry, stirrup is still safe to use!)
I had the same problem and fixed it with an electrical "tape" my dad had. It's not sticky but seals with pressure. It's made to make a waterproof seal on electrical wiring and was by the regular electrical tape at the hardware store. I've had it on there all summer and it still looks like I just put it on. It's really held everything together.