One of King Arthur
's aptly named Knights of the Round Table
in Monty Python and the Holy Grail
Sir Not-Appearing-In-This-Film is merely a picture in The Book of the Movie (actually in the movie) right after Sir Robin the Brave. The picture is the bust of a knight in full dress, with the face removed and the head of a baby replacing it, leaving a large, cavernous space above the head.
Sir Not-Appearing-In-This-Film is aptly named being that he, in no way, plays any part in the hunt for the Holy Grail, much less the actual movie.