Black War Mammoth
The normal black war mammoth mount may carry only one passenger: the player himself. If you want to purchase this mount, you must collect 2,000 honor points from player-versus-player activities such as battlegrounds and then travel to the Wintergrasp zone while your faction holds control of it. Your faction's Wintergrasp quartermaster, found riding on a black war mammoth mount in the upper right corner of the main fortress courtyard, will sell the mount to you.
Grand Black War Mammoth
The grand black war mammoth closely resembles the normal black war mammoth but can also carry two other passengers in addition to the player. Players who wish to ride on this mount as an additional passenger must be in a group with the mount owner and right-click on the mount when standing close to it. To obtain this mount, you must defeat the bosses found in the Vault of Archavon raid instance, which are only available when your faction controls Wintergrasp. Each of the bosses has a very low chance to drop this mount.
Faction Differences
Although the black war mammoths available to the Alliance faction and those available to the Horde faction are identical in function, they present slight color differences. Alliance black war mammoths have grayish armor with a golden border and blue detailing; the armor of Horde back war mammoths is the same grayish color but has a red border and dark red detailing.
Other Mammoth Mounts
In addition to the black war mammoths, other mammoth mounts also exist in the game. Players can purchase two gray-colored mammoths -- the ice mammoth and grand ice mammoth -- after reaching (respectively) revered and exalted reputation with the Sons of Hodir faction in the Storm Peaks zone. The ice mammoth has a single seat while the grand ice mammoth can carry two additional passengers. Vendor Mei Francis in Dalaran sells two brown-colored mammoths: the single-seater wooly mammoth and the traveler's tundra mammoth, which can carry two additional passengers. The latter mount carries two vendors on the additional seats; a player wishing to make the seats available to a group member may choose to temporarily remove one or both of the vendors from the mount. All of these mounts present the same difference in armor coloration as the black war mammoths.