The closest artifact that I could find that has a unicorn horn as part of its make-up is the Sword of Kas, in which only its guards are made of the horn. It does not have any slaying properties on it, however.
I am curious as to what you are looking for, regarding specifics? Are you pulling an item from elsewhere, such as a later edition or a realm that isn't where a DM's world is at? I ask because 1) I am indeed curious and 2) some DMs don't allow some things due to the amount of power they may have.