Mr. Aloysius Snuffleupagus made his debut on Sesame Street in the first episode of the third season. For 14 years, Snuffy was Big Bird's "imaginary friend". None of the adults believed in Snuffleupagus, no matter how hard Big Bird tried to convince them he was real and was standing RIGHT THERE, because there was always a reason they just didn't look until after Snuffy was gone.
In 1985 Snuffleupagus became real and visible to the entire Sesame Street crew. This was for two reasons:
1. the widely reported story was that Sesame Street needed to assure children that they could tell adults about anything, anything at all, without fear that the adult would not believe them. I remember an article boiling this down to the claim that Snuffy was a metaphor for child abuse.
2. The writers were running out of gimmicks to keep people from seeing Snuffy.
Snuffleupagus takes two Muppeteers to operate. He is so heavy that, when a support piece broke and Snuffy collapsed, both Muppeteers were pinned.
But my favorite fact about Snuffleupagus? The same Muppeteer has performed as his butt since 1979.