Wax.
Beeswax is preferred, although you won't be able to drink anything hot out of it (However since hot liquids may not be really good for the horn ANYWAY...). Melt some wax in a double boiler (this is just to ensure that the wax stays liquid long enough for you to coat the inside of the horn), being careful since the wax is flammable. Heat the horn in the oven (150 deg or so will be more than sufficient. Heating the horn is just to keep the wax from hardening as soon as you pour it in). Pour in some of the molten wax, and slosh it around to coat the inside with a thin layer, then pour out the excess, then (with the oven switched OFF) stand the horn upended in the oven with something underneath to catch the drippings. This will ensure a thin coating of wax over the inside of the horn. If putting the horn in the oven makes you nervous, you can heat it by dipping it in boiling water, or with a blow-dryer. With the horn heated up above the temp. of the melted wax, you can slosh the wax around without it hardening instantly, and you should be able to dump out the excess wax. The waxing process can be repeated at later times as you feel it is needed. The bees wax can impart a faint honey-like flavor to beverages, but that's not so bad.