While in the Great Divide, use the Stormfeather's Pride Griffawn Feather that you received for completing Wing Wranglin', hail the griffawn pet and choose "Wanna play a game of fetch?". This is one of five dialogue choices that are used to train your griffawn into a flying mount, and these dialogues can be chosen once each and in any order every 18 hours.
Wait about 30 seconds to find out that your griffawn has not returned
Find your griffawn. Where it is at is very random, so use your map to see the Region of Interest that your griffawn is expected to be at. Tracking for griffawn should work. The griffawn could be on either of the two Great Divide maps, so if you don't see an ROI on your current map, try the other half! Reported locations:
The far end of Storm Gorge, past the Rime camps, at 197, -358, 1422
Along the river on Daggerflow Glacier, between two caves -629.28, -329.77, -83.75 or at -693,-330,-85
At -116, -199, 572 near the Oubliette of the Rime, up a narrow trail that begins around -189, -302, 577
Once you find your griffawn, hail it and the quest will auto-complete.
If you have not done all the side-quests yet: in 18 hours you can pick another training lesson for your griffawn!
If this is your final side-quest: Congratulations! Hail your griffawn pet and you will receive another "Quest Complete!" box where you can pick your new griffawn mount, as well as receive some more experience and money. Choose from: Snow Gryphon, Highland Gryphon or Tundra Gryphon. The Stormfeather's Legacy quest series will now be available from Hircus.