This is the real Sam
Mr. Snuffleupagus first appeared on Sesame Street in episode 0276, the third season premiere. When he first appeared, many of the adults assumed that he was Big Bird's imaginary friend, due to a series of coincidences and near-miss encounters that continually kept Snuffleupaguses and humans apart.
Big Bird would often try to find ways for Snuffy to meet the adults, but something would always cause Snuffy to leave before the humans could see him.
Sometimes, all it would take for the adults to see Mr. Snuffleupagus would be to turn their heads, yet they usually wouldn't do so until Snuffy had already gone.