Snoopy first appeared in the extremely popular Peanuts cartoon strip in 1950. However, his first real appearance might have happened when Charles Schulz sent a drawing to Ripley's Believe it or Not! when he was just 15!
Ripley's Believe it or Not! is a popular newspaper section that had interesting and entertaining bits and pieces of trivia. People would often submit these facts to Ripley they would be published on the newspaper.
A young Charles Schulz wrote to Ripley's about his dog Spike. In the letter Schulz told Ripley that his dog would eat just about everything: tacks, pins, screws, razorblades, stocking and handkerchiefs. The impressive part was that Spike ate, and digested these things, and stayed in perfect health!
Spike the dog would eventually become Sniffy for the Peanuts comic strip. However, because the name Sniffy was already in use by another comic strip dog with the same name, he changed it to Snoopy.
When you look at some of the defining characteristics of both dogs, it's clear how Spike became Snoopy: they both eat anything in sight, and they have similar markings. Do you like Snoopy? You should keep tabs on him on Twitter.