So the same group who figured out about Kosumoth, the Old God world boss also found the Reins of the Long-Forgotten Hippogryph.
The mount is aquired by finding 5 crystals in Azsuna and clicking on them within 8 hours of each other without dying.
A Full List of where those crystals are.
In the water with the basilisks at 54, 33.
North of the wellspring at 37, 32.
In a cave at 50, 20.
In a second cave at 61, 30.
Near the southern cove at 47, 62. There’s no picture of this one, but you can see where it was found below.
Full Map View
If you want to read more about the mount, who found it, and how they came about finding it click the source I used for this information HERE