I cannot express enough how much I adore yaks. A simple, long-haired bovine domesticated and used for millenia in Asia, they are more closely related to bison than to the ubiquitous domesticated cow. Their scientific name - Bos grunniens - literally means grunting ox. Yak are one of the largest of the cow species and can weigh up to a ton. They have the longest hair and densest coats of all the bovine species because they are adapted to the cold of the Himalayas. One of the things I like about them so much is their personality. Where some of the cow species are known to be aggressive (musk ox, Anoa) Yaks are the opposite, they are known for their friendliness and ease of training. I have yet to work with Yak, but maybe someday.
At the Yak Wash - one of the best quests of MoP.
Kun Lai Summit was an interesting and fun zone. The yak's connection to the Grummles only made them more interesting.
Here's a few pictures of Yaks in real life. I think WoW was pretty faithful. The ones in game have one extra set of horns, but that's about the only difference I see.
I cannot share this enough. Wowcrendor - You. Are. Amazing.