The Yaks are first seen in the episode, Second to None, where Twinmasters uses them to lure Second Squad into a trap so that he may take control of them with his chaos energy.
The only difference between the Yak king and the other Yaks is that he is a brighter color and he is a little larger.The Yak's castle is very high up in the sky. It is also the place were Twin masters ambushed Second Squad. On the very top of the temple it appears that there is stone yaks head.
When Second Squad departing from Big Green to go to the
Yak Castle. Ape Trully told them to be careful because the Yaks are very powerful.
In the episode, "The Professional", the Yaks join Big Green along with the Lizards and Flying Squirrels