A Fourth Monkey?

The three monkeys are Mizaru, covering his eyes, who sees no evil; Kikazaru, covering his ears, who hears no evil; and Iwazaru, covering his mouth, who speaks no evil.

Apparently ... there should be a fourth monkey depicted with the three others; called Shizaru, that symbolizes the principle of "do no evil". He may be shown crossing his arms.

Why is it that monkeys always seems devious, creepy or evil?

ps ....(happy birthday dad!!)