The Garn Nighthowl mount drops from Nok-Karosh a level 100 elite wolf with 7717k life in Frostfire Ridge (cords 13.51).
When first released only 1 mount dropped for the entire group so you needed to roll on it for a chance, it appears now that it is a 100% drop for all those in the group, I helped 2 lower level characters kill him yesterday and everyone got the mount with no roll initiated.
He is a pretty easy kill at level 110, but if you want to farm for this mount before you reach 110 then I would suggest finding a group. Making a group in the group finder and announcing it in general area chat to see who else out there wants to join you, If you are level 110 you could do this any way to help out someone else who may be needing it.
This mount can be sold or traded and is the only 1 of the 7 rare spawn mounts in Draenor that can be. The prices aren’t too bad so you can buy it from the auction house if you like.
Nok-Karosh has roughly a 15 – 30 minute respawn.