The origin of Snoopy depicted in this special contradicts the canonical version of how the beagle came to be, whereas Snoopy Come Home (1972) followed the comic-strip's mythos more closely. The main differences: In this special, Charlie Brown, at his current age, wanted a dog to greet him when coming home from school, thus going to buy Snoopy on his own. Originally, when Charlie Brown was 3 years old his parents bought Snoopy for him after a bad day at a playground when a big kid poured sand on him and made him cry. Here, Sally is her current age when Snoopy was bought. Originally, Snoopy was bought before she was born. Here, when Charlie Brown and Linus go to buy Snoopy at the Daisy Hill Puppy Farm, Linus looks at the farm's records and finds that Snoopy was a used dog. Originally Linus found this out by calling the Daisy Hill Puppy Farm when Snoopy ran away to visit Lila in the hospital. Originally, Lila and her family were forced to return Snoopy because they moved into a new apartment where no dogs were allowed. Here, they lived in the same apartment which had a new rule in which no dogs were allowed.
Originally this special was to be a mini-series (much like "This is America, Charlie Brown") depicting Snoopy's siblings, but CBS nixed the idea.
Throughout most of the comic strip Snoopy is said to have been part of a litter of eight puppies. Three of Snoopy's siblings: Andy, Rover, and Molly were created for the special. The others: Spike, Belle, Marbles, and Olaf had all previously appeared in the comic strip. Andy later appeared alongside Olaf in the comic making him the only one of the siblings not created by Charles Schulz to become "officially" part of the Peanuts canon. Molly and Rover, having never officially been accepted or drawn by Schulz in the comics, are not considered canon by most sources and do not appear in any other media.
The only Peanuts animated TV special without Charlie Brown's name in the title.