The Monkey King was once the leader of hozen in ancient Pandaria. He was also a close personal friend of Emperor Shaohao. He played a vital role in Emperor Shaohao's journey to protect Pandaria from the Sundering.
From The Burdens of Shaohao
Though his real name is not known, it was noted in Blizzard's animated shorts "The Burdens of Shaohao" that the Monkey King was a good friend to Shaohao and counseled him in his time of doubt. Approaching the the Temple of the Jade Serpent, there is a tree to the left of the courtyard. The scroll there recalls a tale where the Monkey King gives the Emperor a mask of happiness to suppress his doubt. A sha is then cast out of the Emperor, freeing him forever of doubtful thoughts. The same kind of mask was created and used by the hozen, Ken-Ken while in Zhu's Watch in the Krasarang Wilds.