The achievement is not fully implemented on the PTR, but has the following requirements currently:
Explore all Dragonflight Zones
Reach Renown 15 with all Dragonflight factions
Pathfinder will be coming in 10.2. Its implementation has not been finalized on PTR. More details will be coming soon. Unlocking pathfinder will allow you to use your non-dragonriding mounts in the Dragon Isles.
