With the latest Dragonflight beta build, a new mount has been found: Shellack is the reward for the Glory of the Dragonflight Hero dungeon meta achievement!
The tradition of having a dungeon meta achievement that rewards a mount continues in Dragonflight, with Glory of the Dragonflight Hero requiring you to complete specific boss achievements in the new expansion’s dungeons. Like all other dungeons since Legion, these achievements need to be completed on the Mythic difficulty.