More proof that he was real from the start
One of the earliest episodes involving Snuffy showed him actually walking down the street with everyone there, but, by fate or coincidense, everyone is so wrapped up in there own business that they simply don't notice him. At first it makes you wonder if he realy IS imaginary, because you think "It's imposible for ALL these people to be so busy that they wouldn't notice something as BIG as Snuffy!" But when Luis comes walking down the street, carrying a bunch of boxes, (which are blocking his view), he actually bumps into Snuffy, whitch proves he's real. Luis appologises profusely, and does a turn-about, (which takes him around the big lug), and continues on his way.
Another time is when David is going up to Big Birds door to ask him a question, and he hears the 2 talking. He knows a voice that low couldn't be Big Bird, and he runs to tell everyone. Meanwhile, Big Bird and Snuffy decide to go see a Shirley Temple movie and simply exit the playground area, stage right, without using the door, and by the time David gets everyone to come see, they are both gone. At this point, everyone thinks David is either nuts or was playing a practical joke and leaves the playground area, leaving David flustered and embarased at the door. But right after the door closes, David suddenly hears Snuffy and Big Bird return. Snuffy says "Gee, I'm sorry Big Bird. But I've seen that Shirley Temple movie at least 6 times." Big Bird says "Oh, that's okay Snuffy. I've seen it at least 4 or 5 times myself. I know! Let's go to my nest and watch telivision!" David is listening to all this from outside the door, and is just about to open it to proove to himself that he isn't hearing things, but then simply decides against it and walks away!