After having gone to many fine arts schools by 1906 he had become a very successful illustrator. In 1923 he was introduced to Milne (the author of Winnie the Pooh) and it was then that he created the various human-like animals most of us love today. Milne loved Shepard's work so much he made sure to share a good portion of the royalties and added that when he died he wanted Shepard to decorate his tomb and write the epitaph.
Shepard actually grew to hate Winnie the Pooh because he felt that it overshadowed his other successes.
Christopher Robin was modeled on Milne's son and Winnie the Pooh was modeled on a toy bear of Shepard's son called "Growler".