Updated with a new link at the bottom.
Walt Kelly started his career on the east coast in early comic books, pre-Superman. He then shifted to working at Disney, where he was initially in the story department and later moved to being an animator. His credits include features like Pinocchio, Dumbo and The Reluctant Dragon and shorts like The Nifty Nineties.
He left Disney at the time of the strike and returned to the east coast, where he spent the bulk of the 1940s creating comic book stories for Dell comics of various kinds. One of his strips in Animal Comics developed into Pogo. Starting in 1948, Kelly went to work for the New York Star as an illustrator and art director. He took Pogo along with him as a comic strip. When the Star folded, Pogo found a home in syndication and continued beyond Kelly's death.
In 1966, something happened to Kelly to cause him to send his characters to Mars (though it turned out to be the Australian outback). Perhaps it was boredom or perhaps Kelly was inspired by something, but the 14 month sequence in Prehysteria became the artistic highlight of his time drawing Pogo. The setting allowed him to create fantasy characters and landscapes more elaborate than anything he'd previously done in the strip.
Thomas Haller Buchanan, with the help of Ger Apeldoorn, has created a blog that intends to reprint the entire sequence (with some related sidetrips). The place to start is at the bottom of this page and continue upwards.
Kelly is not to everyone's taste. However, even if you don't share his sense of humour or interest in politics, you have to admire his cartooning chops. His use of the brush is universally admired by cartoonists and his poses are highly influenced by animation, using a character's whole body to communicate the character's emotional state. If you are unfamiliar with Kelly's work, I urge you to take a look.
(Specifically, look at this Sunday page. If that isn't a thing of cartoon beauty, I don't know what is.)