Snoopy, whose fictional birthday has been established as August 10, made his first appearance in the strip of October 4, 1950, two days after the strip premiered. He was first identified by name on November 10. Schulz was originally going to call him "Sniffy" (as described in the 25th anniversary book), until he discovered that name was used in a different comic strip. He changed it to "Snoopy" after remembering that his late mother Dena Schulz had commented that if their family were ever to acquire a third dog, it should be called Snoopy, an affectionate term in Norwegian (the actual term is "Snuppa").
In earlier strips it is not clear to whom Snoopy belongs. For instance, in the February 2, 1951, strip, Charlie Brown accuses Snoopy of following him, only to be told by Patty that Snoopy isn't following Charlie Brown, but merely lives in the same direction. Indeed many early strips show Snoopy interacting with Shermy (who is shown in one early strip running with Snoopy on his leash) and Patty without Charlie Brown, making Snoopy appear to belong to all of the neighborhood kids, similar to the dog Pete in the Our Gang comedies, who is everyone's dog. (Note: in days of yore, it was common for dogs to roam their local area and congregate with local children, and then return to their respective homes). Later, Charlie Brown states that his parents bought Snoopy for him at the Daisy Hill Puppy Farm, after another boy had dumped sand on him while playing in a sandbox.
Snoopy was a silent character for the first two years of his existence, but on May 27, 1952, he verbalized his thoughts to readers for the first time in a thought balloon; Schulz would utilize this device for nearly all of the character's appearances in the strip thereafter. At first, Snoopy acted as a normal dog, and would only think in simple one-word phrases (such as "FOOD!"), but then became more articulate.
In addition to Snoopy's ability to "speak" his thoughts to the reader, many of the human characters in Peanuts have the uncanny knack of reading his thoughts and responding to them. In the animated Peanuts films and television specials, Snoopy's thoughts are not verbalized; his moods are instead conveyed through growls, sobs, laughter, and monosyllabic utterances such as "bleah," "hey," etc. as well as through pantomime. Unlike a regular dog, other characters treat him intelligently by taking directions from him, incorporating him into the local baseball team, and so on despite his inability to speak (this anthropomorphic tendency increased as time went on).
The only exceptions are in the animated adaptations of You're a Good Man, Charlie Brown and Snoopy!!! The Musical, in which Snoopy's thoughts are verbalized through voice overs (by Robert Towers and Cam Clarke, respectively). Animation producer Bill Meléndez voiced both Snoopy and (eventually) Woodstock in numerous television specials from 1965 to 2006. In Peanuts Motion Comics, Snoopy's thoughts appear onscreen as text in thought bubbles, without voice.
Oddly enough, the first time a beagle was mentioned in the strip (December 5, 1960), Snoopy denied being one. As Snoopy dozed, Charlie Brown paraphrased Gertrude Stein: "Beagles on the grass, alas." To this, Snoopy replied, "I ain't no stupid beagle!" (Years later, Snoopy would paraphrase the Stein expression himself: "Pigeons on the grass, alas; Dogs on the ground, abound." (June 25, 1982)
As the series progressed, Snoopy became a more human-like dog. His character is that of a dog who pretends to be a person (or who sometimes forgets he is a dog). In one Peanuts strip, Sally had to do a report on animals for school, and requested Snoopy's help. But Snoopy was reluctant. "How can I help?" he thought. "I don't know any animals."
Many of Peanuts' memorable moments come in Snoopy's efforts as a novelist: his eternal opener on the typewriter "It was a dark and stormy night..." is taken from Edward George Bulwer-Lytton's 1830 novel Paul Clifford. Almost all his submissions are rejected by potential publishers, who eventually resort to rude dismissals and cruel jokes to attempt to prevent being bothered by Snoopy. The contrast between Snoopy's existence in a dream world and Charlie Brown's in the real world is central to the humour and philosophy of Peanuts (e.g., the Peanuts book title Life's a Dream, Charlie Brown). "It Was A Dark And Stormy Night" remains his most successful work.
Schulz summed up Snoopy's character in a 1997 interview: "He has to retreat into his fanciful world in order to survive. Otherwise, he leads kind of a dull, miserable life. I don't envy dogs the lives they have to live."
---excerpt from wikipedia---