I wish I had better provenance on this piece ... but it came from a reputable auction house (North American Auctions) and no doubt it is bison horn. I also think the representation that it is Sioux is also accurate.
There are several natives that did bison horn reproduction carvings over the past 30 years; the horn caps being available pretty readily in South Dakota. It's not a real old piece, but it is traditionally done and well done. Very nice polychrome quill work and deer leather drop, as well. With case (and the Palo Duro Canyon petrified wood that holds it upright).