I just did this on a knife this morning! Sanded with 400 and 600 grit sand paper, then took it to a sewn cotton wheel with white rouge on it. I masking taped over the blade and the Ho wood of the handle so I would just buff the horn. Worked nicely; not a full shine, but looked much better than it was before. If you don't have a lathe/buffer, get a small buffing wheel (like 6" or so) from a hardware store, white polishing compound and an adapter so you can use the wheel in a cordless drill or drill press
The adapter is a piece of threaded rod with a smooth end and some washers and a nut to tighten on the wheel. If using the cordless drill, I would put the drill in a vise or have someone hold it so you can use 2 hands on the knife.