Mountain o' Mounts is the second from last achievement of a series from the same kind.
Video guide Edit
Alliance, Horde, Cataclysm
You receive the mount in the mail, along with the following message:
From: Mei Francis
Subject: Mountain o'Mounts
I've heard your stables are nearly as extensive as mine, now. Impressive! Perhaps we can help one another...
I've one too many dragonhawks, and hoped you could give this one a home. Naturally it's been trained as a mount and not a hunting pet, and you'll find it as loyal and tireless as any other steed I raise.