Hi,
Just wanted to share for the people that are trying to get their hands on this mount, the time I had just got it myself, so you can have an estimate of when it will respawn again (between 2 to 4 hours respawn timer more or less).
I just caught it today at 1:03pm Sargeras server time. So the next one will probably spawn between 3:00 and 5:00pm server time.
Hope it helps.