"To help easy storage concerns in the new expansion, Blizzard is
implementing an entirely new system in which all Vanity Pets and Mounts
and learned like spells and stored in an interface much like that of a
Hunter's pets. This interface is accessible off of the Character sheet.
(See the image at left.)

From the official EU Beta Forums:

We’re pleased to announce a convenient upgrade to the way that mounts and vanity pets are handled in Wrath of the Lich King.

Players will be able to “learn” a mount or non-combat pet much
like learning a spell, recipe, or new ability, and these creatures will
then show up on a new Pet tab within the Character Info section of the
interface. Players will be able to access and preview their learned
mounts and vanity pets through this tab.

Once learned, the pet icon or mount icon will no longer appear
in bag spaces or bank spaces. This inventory space will be made
available once again for other adventuring needs. Pets can still be set
to hotkeys by dragging them to the hotkey bar, much like any other
spell or ability"

 not my own words <.<

 =_