He became visible a very long time ago, back when my children, now grown, were watching the show. I remember the episode, the onscreen suspense leading up to it, and the characters’ reactions afterwards. I was unhappy about it as a viewer and as a parent because
(a) it messed with the one of the running themes,
(b) it seemed to say you shouldn’t talk about imaginary things after all—you have to be able to prove they’re real,
(c) for that reason it seemed to turn prior messages on their head, and
(d) the critter himself was the dumbest-looking and most unattractive character on Sesame Street.