The venerable puppets of Sesame Street have gone through many iterations throughout the years — don't forget that Mr. Snuffleupagus was pretty unsettling in his first appearance. And before characters like Big Bird and Cookie Monster made it to the screen, they were but mere scribbles in Jim Henson's notebook.
Clockwise from the top-left, that's Cookie Monster (who began his career as a corporate spokesmonster in the 1960s), a chicken-like Big Bird, early Elmo, the antecedents of Ernie and Bert, and "the Gourmet Bird" from a never-filmed Stouffer's Food television spot, who served as a proto-Big Bird of sorts. (Click on the above image to expand.)
And below we have another early Big Bird sketch (famed Big Bird puppeteer Carol Spinney apparently said of this first puppet, "He didn't look too keen. I thought he was one of the ugliest things I'd ever seen."), a semi-disconcerting Oscar the Grouch, and Sam The Robot (more info on Sam's development here). For more behind-the-scenes Muppet fun, here's David Bowie spinning his crystal balls in Labyrinth and Snuffy eating a child.