Reins of the Long-Forgotten Hippogryph is a new Hippogryph mount added in Legion. The Hippogryph have not been active in wow, but the mount model has been added to the database long ago. And finally, Blizzard remembered the Hippogryph and added it to the game.
It is a Legion secret since no information there to show where and how to get the newly added Reins of the Long-Forgotten Hippogryph in wow game. Similar to Fathom Dweller reward from Kosumoth the Hungering, players who are enthusiastic about puzzle found the method to obtain Reins of the Long-Forgotten Hippogryph:
Ephemeral Crystal located in Azsuna and spawn at once in random points. When crystals spawn and you click any of them, you got buffed and now have 8 hrs to find other ones. It even timing when offline. If a player who first finds all five crystals will obtain the mount, that all crystals and other players' buff will disappear. Then next round. If you die, you have to collect all five crystals again to obtain the Long-Forgotten Hippogryph. That is, every Crystal spawn is a contest to contend the first one collecting all five crystals(no order, but click all 5).
Like the Fishing Contest in Stranglethorn, the first player who collected all five crystals will gain the Long-Forgotten Hippogryph. Seems also need a bit of luck to get the mount.