This archaeology mount is only available twice a year (in Feb + August), via a quest “Laying to Rest” offered in new Dalaran, and the quest is only open for 2 weeks.
Check at the archy trainer in Legion Dalaran to see when the quest is up this 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! If you missed out on other moose mounts then move on this fast! If you are into RP it is Huln’s mount Eche’ro who helps you in the Huln scenarios in HM in 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 110 or above is easy).