Reins of the Grand Expedition Yak

Reins of the Grand Expedition Yak is a mount containing the repair vendor Cousin Slowhands, and the transmogrifier Mystic Birdhat, and is the only known mount in-game to offer portal transmogrification services.

Other vendor mounts including Reins of the Traveler's Tundra Mammoth and the Reins of the Mighty Caravan Brutosaur, the latter offering portable Auctioneer services.

Source
This item is purchased from Uncle Bigpocket in Kun-Lai Summit for, without any skill or quest requirements, and is not discounted by Best Deals Anywhere, the goblin racial trait.

Obtaining this mount awards the Grand Expedition Yak (achievement) achievement.

Mount Journal
These beasts of burden are known to carry over five times their own weight and to be capable of lasting several days without food or water.