Ancient monkey is a transmogrification character. Although the player can transform into them, the NPC itself can never actually be seen in-game.
The player can transform into this character by wielding an ancient gorilla greegree, making them resemble an ancient gorilla.
Trivia
The NPC was first released with Monkey Madness I in 2004, but it could not be transformed into; the monkey skull required to make the greegree was locked behind a door until an update in 2015.
Its model is nearly identical to that of an elder guard, except it has a blue face.