Make friends with a druid who has (or is working on) Swift Flight form. Run Heroic Sethekk Halls with the druid to summon the optional Raven God boss. The mount drops from him.
The last quest in the Swift Flight chain provides the druid the item to start this encounter (in the room before the last boss, IIRC) in order to get their form. It can be summoned again on later runs after finishing the line.