Drysc, from Blizzard, informed the following:
The swift (armored) nether drake which is a seasonal arena reward will be a 310% speed flying mount.
The (unarmored) nether drake will be obtained through a quest (which is planned to be implemented in a future patch) and will be a 280% speed flying mount. This nether drake is intended to supply players with a cool new epic mount and won’t require any tasks within raid encounters. It’s a reward that all players can work towards, but it will still be difficult to obtain.
There is another mount that is 310% speed that is yet undiscovered which is intended to be an equally rare and difficult to obtain equivalent to the arena reward. That’s the only information we’re able to give at this time.