Here's what my source had to say, wofa:
The "three wise monkeys" or three mystical apes ("sambiki saru") are the attendants of Saruta Hiko no Mikoto or Koshin, the God of the Roads. They are (1) Mizaru, covering his eyes, who sees no evil; (2) Mikazaru, covering his ears, who hears no evil; and (3) Mazaru, covering his mouth, who speaks no evil. The source that popularized this pictorial maxim is a carving over a door of the Sacred Stable at the famous temple in Nikko, Japan (carved in the 17th Century). However, the maxim probably originally came with Buddism from India via China in the 8th Century (associated with Vadjra who taught that if we do not hear, see or speak evil, then we will be spared evil). The “fourth” mystical ape is simply a corrupt invention of the tourist trade and may be seen occasionally in humorous netsuke as “know no evil” (“Iwazaru”) or “do no evil” (“Shizaru”).
http://www.worldcollectorsnet.com/netsuke/messages/462.html I was very surprised by finding this; I've heard that expression all my life, and it never once occurred to me that these creatures might have names.