Sleipnir is a paid mount, an extremely popular one in sales, and its price justifies its priority over every other current obtainable mount to be flyable in this game.
Sleipnir lore in the FF series is flyable, in actual mythology is flyable.
They stated in a LL they were going to do this to Sleipnir. (must be grouped with the male bunny costume)

Please Help.
Thanks !
I didn't find the right solution from the internet.
References:
-http://forum.square-enix.com/ffxiv/threads/337490-2.0-primal-mounts-will-they-ever-fly
- 3d animated video Examples