The Aerial Unit R-21/X was added to World of Warcraft in patch 8.2, and it is tied to the new Mythic-only Dungeon "Operation: Mechagon". This dungeon consists of 8 bosses, and the Aerial Unit R-21/X drops from the final boss, King Mechagon. The mount is a flying mount, but it can be used as a ground mount as well.
The dungeon also has another mount on the drop table, because you have a hance to get the Machagon Peacekeeper mount from the 4th boss as well, HK-8 Aerial Oppression Unit.
The drop rate for the Aerial Unit R-21/X seem to be around 1 % like the rest of the Mythic-only mounts that were added to dungeons in Battle for Azeroth, and only one person in the group will get it, so it's not for the entire group if it drops. It is tradable to other members of the group, so it's possible to give it away if you are feeling nice.
It's worth noting that the mount will only drop on Mythic difficulty since Operation: Mechagon will only be available on this difficulty. However, the dungeon is expected to open for other difficulties such as Heroic and Mythic+ after the 8.3 patch, so it might drop on these difficulties as well once we get to that point. It seems unlikely that it will drop on Heroic since no of the other Mytic dungeon mounts do, but it does seem likely that you will be able to get it from Mythic+ caches once that makes it to the live game.