Summon trigger: http://www.wowhead.com/spell=215345/sum ... hippogryph
NPC: http://www.wowhead.com/npc=108366
Mount item: http://www.wowhead.com/item=138258
Learned mount: http://www.wowhead.com/spell=215159
Although it may make it easier, I'm hoping its not some rare Class Hall mission similar to the Llothien Prowler.
Theres a certain high mountain in Azsuna that has my attention as a place for this NPC. The path up wasn't obvious (to me anyway), and at the top of the long winding path is a pavilion with a couple NPC that summon a rare felbat. Thats all I can tell happens up there, and its not an insignificant chunk of map. Heres the map with 3 yellow X where the points of interest I found are.

First X (south west): This is an odd circle of stones right off the side of the path. Nothing here that I've found.

Second X (east): Abandoned horde painting items. This is neat but I don't think its relevant. Probably flavor. It does make for a good view though.

Third X (center north): This is, to me, the most interesting. Its not directly off the path, you have to wander a little ways down. The path leading up to this mountain is full of neutral cliffwing hipoogryphs and llothien foxes that run around. The way the foxes run made me think they were leading you somewhere, but now I don't think they are. Anyway, all the wildlife up here is neutral except these two hostile buggers.

These are Bloodthirsty Foxes, and from what I can tell, these are the only two in the game. They are programmed to stare at this tree. Every minute or so they will both walk up to it then run back. Theres nothing there, its not like they have prey cornered. They are staring at a blank trunk.
You can climb the tree. And I have, multiple times. I've camped in it, visited it at odd hours, hoping to see maybe a nest or something that would trigger the Long-Forgotten Hippogryph. I've jumped from it with a glider thinking theres something I can only reach from here. I've killed these foxes over and over (they are hostile afterall). Nothing so far.
Besides these, I have ran all over this mountain looking for clues. I even went back to Feralas where the model was first introduced to look for clues. Nothing is jumping out to me. MUST FIGURE THIS OUT.