Didn’t he also voice the animated Dr. Know in Artificial Intelligence? IIRC he was uncredited but that was some time ago.
He probably did a lot of them for fun, but IIRC the reports about Munchasuen were that his agent demanded his name not be credited because the production was very troubled (like many Gilliam movies) and the agent feared the studio would try to cash in on Williams’ name to help salvage some box office on what was looking like a dud. (The movie did flop at the box office).
In John Huston’s “The Kremlin Letter” (1970), an unknown, unidentified actor plays the character Polyakov, who is being paid to retrieve the document. He has a rather lengthy scene at the beginning, but no clue as to who he is.
The young “Tomorrow Belongs to Me” singer in “Cabaret” is uncredited, although numerous sources to attest to the identity of the actor.
Don’t forget Whoopi Goldberg going uncredited in the Star Trek: TNG movies.