Reins of the Black Riding Yak

The Reins of the Black Riding Yak was meant to be sold by Uncle Bigpocket at the Grummle Bazaar in Kun-Lai Summit but never made it out of the Mists of Pandaria beta.

Mount Journal
Grummles traveling Kun-Lai Summit by night prefer to ride this dark-haired yak to remain unseen, though they remove the bells from its caparison first.