This guy is one of the Hunter Pet Hidden Tames available in the game.
The item to make him spawn is the Spectral Feather and it can be found in the pile of hay near the flight master in Stormwind City (cords 72.72) It has a respawn of 12-14 hrs.
This pet can only be summoned by a level 110 Beast Master Hunter but any Beast Master Hunter can tame him you just need a friendly 110 to summon him for you.
Lost Spectral Gryphon is available to both factions all you have to do is get to him. The easy approach is to fly in and land you will die but once you run back, stay dead until the feather spawns, you can see it in death so you won’t miss out.
Once spawned resurrect, click the feather, pop aspect of the turtle and tame him… you may die again but the turtle should keep you up long enough for it not to matter 🙂
Do keep in mind however that even though only a hunter can summon him anyone can kill him so keep your eye out for nasty pet killers.
Check out the other Hidden Tames below.