In fact, Spike was a real dog; a mixed-breed, black and white pup owned by Charles Schulz in his youth.
Spike was the subject of "Sparky" Schulz’s first published cartoon which the 15-year old sent to "Ripley's Believe it or Not" The cartoon, shown below, noted his dog’s gastrointestinal feats. Schultz claimed Spike understood 50 English words.
Snoopy was not in the very first Peanuts comic strip, which ran on October 2, 1950. Instead, he showed up later, and in a form very different than he looks now. The softer, more-rounded Snoopy that we know today did not show up until the mid 1960s. Spike, Snoopy's brother, did not show up until about a decade later.