This is a tanned Icelandic horse tail.
This horse tail has been de-boned, scraped, washed, and tanned. The base of the tail has the original skin where the hair is attached.
Many Native Americans use horse tails as hair on their masks. They are also used on carousel horses, for making Halloween costumes, and by costume designers for all sorts of exotic looking outfits.
Horses are not an endangered species. No horses are killed just for the tails. They are not subject to CITES or USFWS controls.