I'm 99% sure this isn't a mount.
It appears when the aspects charge the Dragonsoul in the Madness of Deathwing encounter and Thrall fires the beam of doom to deliver the final blow to Deathwing.
EDIT: http://www.youtube.com/watch?v=xmd7G...ailpage#t=762s
You can vaguely see it in the golden transparent orb in that video.