Gruesome Flayedwing

The flayedwings of Maldraxxus are prized for their ability to soar effortlessly through the skies of the Shadowlands, allowing their rider a surprise opportunity to strike at their foes or support their allies.

Riding Requirements:

This mount is available to all eligible characters on your account.
Level 10
Apprentice Riding (ground)
Expert Riding (flying)

Source:

Abomination Factory (Necrolord Covenant) in Butchers Block, Maldraxxus. Requires Exalted with Stitchmasters.
The Ember Court (Venthyr Covenant) in Revendreth.
Queen's Conservatory (Night Fae Covenant) in Ardenweald.
Path of Ascension (Kyrian Covenant) in Bastion.

Notes:

This mount can be obtained in different ways depending on which of the four Shadowlands Covenants you're a member of. In each case it requires the use of your covenant's special feature (below), which must be activated and upgraded with Reservoir Anima:

Venthyr Covenant - The Ember Court. Chance loot from tribute chests at the end of your Court session, possibly only when Alexandros Mograine or Baroness Vashj are guests, and their happiness probably has to reach Elated. This requires the Court to be upgraded to rank 3 or 4.
Night Fae Covenant - Queen's Conservatory. Chance loot from the reward cache when you use an "Martial" type spirit and there are four Wildseed Root Grains active in the adjacent catalyst plots. There's only one plot that fulfils this criterion and it requires the Conservatory to be upgraded to rank 5.
Kyrian Covenant - Path of Ascension. Can be purchased from Binkiros for 2500 Reservoir Anima after you've completed the achievement, All the Colors of the Painbow. This requires the feature to be upgraded to rank 2.
Necrolord Covenant - Abomination Factory. Can be purchased from Atticus for 5000 Reservoir Anima after you've reached Exalted with the associated faction, Stitchmasters. This doesn't require the feature to be upgraded beyond rank 1, although your reputation will increase more quickly with higher ranks.