The Reins of the Long-Forgotten Hippogryph is a new mount added in the Legion expansion pack for World of Warcraft, even though the model was actually first discovered several years ago in the Burning Crusade expansion. However, the Hippogryph finally made it into the game, and is found by finding and clicking on 5 different Ephemeral Crystals in Azsuna.
It sounds easy, but much like the other rare spawn mounts from earlier expansions, it is much harder than it sounds. Firstly, the five crystals have a random spawn timer, so you never know when they will appear. Also, only the first person to find and click on all the five crystals will get the mount!
So, in order to farm this you first need to find a crystal. If you find one, the four others will also have spawned, so this is your chance to get the Reins of the Long-Forgotten Hippogryph. Clicking on each crystal gives you a buff, so make sure you do not die before you find all five, or else you need to reclick on them. Luckily they don't disappear when you click on them, so it should be fast to find them again.
The crystals has lots of different spawn points, so it's not possible to just go for a certain route. You need some luck in order to be the first to find all five, and once a person does so, all the crystals disappear, and you need to wait for a new spawn before you get another chance.
If you manage to click on all five crystals, the Reins of the Long-Forgotten Hippogryph will be automatically added to your inventory, so just right click on it to add it to your mount collection.
Here's a nice TomTom macro you can use to add all the known waypoints. If you find a crystals, you should run this and check the other locations! Just be aware that not all waypoints are currently known by the community, so some of the crystals might even spawn in different locations.
/way Azsuna 54, 33 Nar'thalas academy crystal
/way Azsuna 37, 32 Nor'danil wellspring north crystal
/way Azsuna 50, 20 Azurewing repose cave crystal
/way Azsuna 61, 30 Cave crystal
/way Azsuna 47, 62 Oceanus cove crystal
/way Azsuna 42, 8 Lost Orchard crystal
/way Azsuna 34, 35 Nor'danil wellspring south crystal
/way Azsuna 47, 33 Azurewing repose south crystal
/way Azsuna 53, 28 Liothien crystal
/way Azsuna 59, 38 Hatecoil warcamp crystal
/way Azsuna 67, 33 Felblaze Ingress east crystal
/way Azsuna 60, 53 The Ruined sanctum west crystal
And a little funfact for the ending: the mount was first discovered in the patch 2.3 datamine all the way back in 2007! Here is the original MMO-Champion article with the first discovery.