WHAT IS IT?
A Yak is a long haired bovine that has long being domesticated by mankind but also maintain an outreach program to their more wild cousins.
It would be easy to simply say that a yak is like a cow that got fired and hasn't seen the point of shaving for a few weeks but I would be forced to point out that cows don't shave 
WHERE DO YOU GET IT?
You'll need to wrap up warmly since proper specimans are located in the Himalayan region of South Central Asia, The Tibetan Plateau and as far north as Mongolia and Russia where it can get a wee bit chilly.
USING IT IN REAL LIFE:
Having obtained a yak you might be wondering exactly what to do with it. This is where having a plan comes in handy. You can't just steal a yak on a whim without knowing what to do with it, how to get it past customs and a myriad of other yak related problems that may have already cropped up. 
Obviously the biggest problem is getting it back home. While the first solution might be to do what the drug smugglers do and swallow it I should point out that this is a very uncomfortable method and one that is, thankfully, unecessary.
With a can of paint and some wooden framework any yak can quickly and easily be transformed into a working replica of a russian automobile built at the hieght of Soviet power. As long as you have the correct papers you'll easily be able to bluff your way past any authorities and ride your yakmobile home. If you must cross an ocean or similar body of water then take the can of paint and change the registration to match that of a Nucular submarine  after that it's a pleasant jaunt on the ocean blue
USING IT IN WRITING:
The easiest role that a yak might take is as an interesting and hardier alturnative to horseback. It might be that the dreaded yakriders of Bingybangyland are particularly fierce 
Perhaps the yak is venerated in certain parts of the world and your characters have caused a bit of a faux pas when they butchered one and served it up to a person of note?
What if this wasn't a mistake? 
MIGHT ONE TAKE OVER THE WORLD WITH IT?
- Yes it's an easy one but there are very few jokes that one can make about the Yutyrannus huali (a feathered tyranosaurus)."We don't get many Yutrannus huali in here""At these prices I'm not suprised"
- Except in the most extreme of circumstances.
- Isn't it lucky that you've got me to think of this stuff for you?
- The hippies won't like it but you won't care. You've got a nucular yak! Good for you!
- Due, in no small part, to the fact that they are the dreaded yakriders of Bingybangyland.
- A cheap shot I know but I had to sit through dinner with vegans last night and the amount of moaning about what they couldn't eat had me seriously considering this option.