Munki-Gu is an ancient statue that was hidden in a secret chamber inside the Mystery Mountain cave. It has the power to change the appearance of others to look like someone else.
In "PJ Party Mountain," Night Ninja uses the Munki-Gu statue to change his appearance to An Yu so he could trick the PJ Masks into breaking all the rules. An Yu also uses it to change her appearance to Night Ninja to prank the team.
The Munki-Gu statue was created when Munki-Gu was trapped in Mystery Mountain for 1,000 years for being too naughty, changing him to a stone statue.
Eventually in the future, Munki-Gu will make another appearance which shows him being transformed back to a normal monkey from his statue form.