Drop: World DropLocation: Temple of Ahn'Qiraj
This item is no longer available in game.
This item is currently unobtainable on Live servers, because it was a reward for the first person to complete the quest Bang the Gong! on each server. Currently, each new server to open will already have the Gates of Ahn'Qiraj open and the questline to open the gates was removed from the game in Cataclysm.
I have personally never seen this mount, and I even have no idea who is the Scarab Lord on my server. However, there undoubtedly is no rarer mount than this.