Snoopy appeared on October 4, 1950, two days after the first Peanuts strip. He was originally Patty's dog before being adopted by Charlie Brown. He was called Snoopy for the first time in the November 10th strip. On March 16, 1952, his thoughts were first shown in a thought balloon. Snoopy first appeared upright on his hind legs on January 9, 1956, when he was shown sliding across a sheet of ice after Shermy and Lucy had first done so.

Source: Wikipedia