Apparently in the early seasons of Sesame Street, Mr. Snuffleupagus was only seen by Big Bird. Gordon, Susan, Mr. Hooper, Grover, not even grouchy Oscar caught a glimpse of this giant, cocoa wooly mammoth. The problem was that Mr. S would get in all kinds of trouble like eating someone else's cookie or breaking the letter J. Then he’d sneak off leaving Big Bird to take the blame. This went on ‘til episode 2096 (aired November 18th, 1985). The plot had Big Bird tricking Snuffy into revealing himself to the entire neighborhood. Big Bird was finally vindicated.
Incidentally, that episode was brought to you by the letter L.