
” The Fallen Charger is a new rare enemy that can be found anywhere in the Maw and has a chance to drop the item Fallen Charger’s Reins, which grants you the mount. The rare’s spawn location varies greatly and may depend on which Covenant Assault is active at the time—though it’s currently unclear whether an Assault needs to be active for it to appear. The most significant number of sightings according to comments on Wowhead have been in the Maw’s ‘main’ area, rather than The Beastwarrens. It also seems to have a respawn timer of around eight hours.”
Source: https://www.pcgamer.com/wow-fallen-charger-location-rare-mount/