This archaeology mount is only available twice a year (in February and August), via a quest “Laying to Rest” offered in Legion Dalaran, and the quest is only open for 2 week each time.
Check at the archy trainer in Legion Dalaran or check Wowhead news in mid Feb/Aug to see when the quest is up that month and be sure to turn the quest back in before it expires to get the mount.
It is a spirit moose, and now a flying mount. Looks just like the spirit moose rare hunter pet so a great combo! And it fits well in Shadowlands. If you missed out on other moose mounts then move on this one! If you are into RP it is Huln’s mount Eche’ro who helps you in the Huln Highmountain scenarios in Highmountain in the Legion questline.
Once you pick up the quest from the archy trainer in Legion Dalaran, you need to get 600 bone fragments from archaeology digs in Highmountain and then turn the quest back in to her before it expires and the mount is yours. The bones drop along with regular archy shards on each dig, plus every 10 digs or so you fight a ghostly HM tauren Angry Spirit for 13-15 more bones.
Having done it on two tanks, it takes about 2-3 hours or so of digging. Once accepting the quest, dig sites in HM will spawn continuously for the alt with the quest (usually in mob piles of basilisks, drogbar, jormungar serpents, Ettins and harpies but at Legion 110 (now 45 in SL) or above is easy). Feel free to ignore a site you dislike as it will spawn more as long as you complete at least one site that is up.