The Reins of the Ancient Frostsaber (previously the Reins of the Frostsaber) teach you how to summon an Ancient Frostsaber mount. This item could be purchased for 1,000 from Lelanai <Saber Handler> in Darnassus prior to patch 1.4. With that patch, most level 60 faction mounts were replaced with new, armored versions. Players who had one of the original mounts had the option to trade in for a new mount or keep the old one.