Several rare mounts have appeared thanks to free flight in Outland. Thus arrived new rare flying but also terrestrial mounts.
Reins of the Raven Lord
Reins of the Raven Lord can be obtained from Anzu in Halls of the Sethekk on Heroic difficulty. This mount drops at the rate of 1%. The mount grants you the Reins of the Raven Lord stunt.
Swift White Hawkstrider
The Swift White Hawkstrider is a sublime mount that can be obtained from Kael'thas Sunstrider in the Magisterium's Terrace on Heroic Difficulty only (Dungeon). The mount drops to approximately 4% and, once learned, grants you the Swift White Hawkstrider stunt.
Reins of the fiery warhorse
The Reins of the Burning Warhorse are certainly one of the most popular mounts for players. These can be obtained from Attumen the Huntsman in Karazhan. The mount drops to around 1% and grants the Reins of the Burning Warhorse feat.
Ashes of Al'ar
Finally, arguably the most majestic flying mount in this expansion, the Ashes of Al'ar, a sumptuous phoenix that can be obtained from Keal'thas Sunstrider in Storm Dungeon (Raid). The mount will grant you the Ashes of Al'ar feat and the mount drops at the rate of 2%.
It only remains for me to wish you good luck and good luck if you go hunting these sumptuous mounts!