Mammoth mounts are Epic vanity mounts that were added in Wrath of the Lich King. Some can accommodate passengers, and they all (currently) have subtle (and sometimes not so subtle) shadings for each faction.
Mammoth mount types
Exotic mounts in Dalaran
The Traveler's Tundra Mammoth is also available from Mei Francis. They cost 20,000. This price is reduced by any Kirin Tor reputation adjustment. They carry two vendors, who can be kicked out (individually) to allow room to carry passengers.
There are two sets of Mammoth mounts available from Wintergrasp.
Secondly, there are the Grand Black War Mammoths. These drop from all the bosses in the Vault of Archavon, but only rarely. These mammoths can carry two passengers.
Sons of Hodir mammoths
Additionally, exalted characters are offered the Grand Ice Mammoth for 8,000 (10,000 less faction reputation adjustment). As with the Traveler's mammoth and the Grand Black War Mammoth, this mount has space for two passengers.