In previous expansions, some final Mythic final bosses would drop a unique mount. However, in Dragonflight with Dragonriding mounts, Blizzard has opted to go for amazing Dragonriding customizations instead. In Vault of the Incarnates, the Dragonriding Customization was Renewed Proto-Drake: Embodiment of the Storm-Eater, but it dropped on any difficulty.
In Aberrus, the Highland Drake: Embodiment of the Hellforged seems to only drop on Mythic difficulty as the Dungeon Journal was recently hotfixed.
