Black Riding Yak

The only Yaks available atm are:

• 
  
• [item=87789] (3k)
• 
  
• [item=87788] (3k)
• 
  
• [item=84101] (which costs 120 freaking thousand)
• 
  

You can buy any of them from [npc=64518] at Kun-Lai Summit.

As for this one, the Modest Expedition Yak, and the Kafa Yak, they seem not to be available. Yaks were intended to be Pandaren mounts (instead of the turtles). No idea why they backed up, but looking and how funny my yaks walk, I'm guessing they had some issues with the model.

This mount can be bought from [npc=219032] for [currency=2778] in any Infinite Bazaar during the [event=1525] event.
He sells a total of 41 mounts, of which 30 are unique to [event=1525].

Timerunning Additions:
Total amount needed for the new mounts: 103.400 [currency=2778]
[item=213582] - 2200 [currency=2778]
[item=213576] - 2200 [currency=2778]
[item=213584] - 2200 [currency=2778]
[item=213595] - 2200 [currency=2778]
Jungle Riding Crane - 2200 [currency=2778]
[item=213602] - 2200 [currency=2778]
[item=213603] - 2200 [currency=2778]
[item=213604] - 2200 [currency=2778]
[item=213605] - 2200 [currency=2778]
[item=213606] - 2200 [currency=2778]
[item=213607] - 2200 [currency=2778]
[item=213608] - 2200 [currency=2778]
[item=213609] - 2200 [currency=2778]
[item=213627] - 2200 [currency=2778]
[item=213628] - 2200 [currency=2778]
Black Riding Yak - 2200 [currency=2778]
Modest Expedition Yak - 2200 [currency=2778]
Kafa Yak - 2200 [currency=2778]
[item=213626] - 4400 [currency=2778]
[item=213624] - 4400 [currency=2778]
[item=213625] - 4400 [currency=2778]
[item=213623] - 4400 [currency=2778]
[item=213622] - 4400 [currency=2778]
[item=213621] - 4400 [currency=2778]
[item=218111] - 4400 [currency=2778]
[item=213600] - 6600 [currency=2778]
[item=213601] - 6600 [currency=2778]
[item=213596] - 6600 [currency=2778]
[item=213597] - 6600 [currency=2778]
[item=213598] - 6600 [currency=2778]

Original Mist of Pandaria mounts:
[item=94229] - 18700 [currency=2778], drops from [npc=69769]
[item=87777] - 18700 [currency=2778], drops from [npc=60410]
[item=104269] - 38500 [currency=2778], drops from [npc=73167]
[item=89783] - 38500 [currency=2778], drops from [npc=62346]
[item=94228] - 38500 [currency=2778], drops from Oondasta
[item=95057] - 38500 [currency=2778], drops from [npc=69099]
[item=87771] - 38500 [currency=2778], drops from Sha of Anger
[item=93666] - 38500 [currency=2778], drops from Horridon
[item=95059] - 38500 [currency=2778], drops from [npc=69712]
[item=104253] - 38500 [currency=2778], drops from Mythic Garrosh Hellscream
Reins of the Thundering Ruby Cloud Serpent - 50000 [currency=2778], drops from [npc=64403]

Actually, you are wrong: About two weeks before the beta was launched (hence, with most models already done) they released an art concept, said that there was a wink at the pandaren racial mount in the drawing, and encouraged players to guess what was it. Shortly before the release they confirmed they were talking about the yaks.