can anyone advise me on what to use as a preservative on a very old set of antelope horns?

Question

these horns are over one hundred years old and deterorating badly.

Answer 1

Source 1 has some tips but basically suggests you consult a professional. Source 2 recommends linseed oil for antlers but not horns. Sources 3 and 4 are amateur advice, specifically on antelope horns. You might describe your situation to that forum (taxidermy.net). Good luck; I think you'll need it. If all else fails you might try a transparent, rigid coating or filling; epoxy or anything better you can think of.