Modest Expedition 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 Black Riding 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]

This is one of the mounts on the offical WoW armory that has currently never been implemented into the game.
In the table below I've listed all the mounts that appear on the WoW armory but have never been implemented into the game, patch indicates the patch it was datamined.

Mount

Patch

Illidari Doomhawk

3.0.3

Black Polar Bear

3.0.3

Black Riding Yak

5.0.4

Blue Qiraji War Tank

7.3.5

Modest Expedition Yak

5.0.4

Gray Wolf

?

Brewfest Kodo

3.0.3

Red Qiraji War Tank

7.3.5

Riding Kodo

2.4.3

[spell=302796]

8.2.0

[spell=302794]

8.2.0

[spell=239363]

7.2.0

[spell=302795]

8.2.0

[spell=302797]

8.2.0

[spell=194046]

6.2.1

Tiger

3.0.3

Kafa Yak

5.0.4

[npc=359]

?

Grand Caravan Mammoth

3.0.1