Kermit the Frog was during the early days of the Muppets a different being altogether. The character was introduced in 1955 on the Sam and Friends show and he had round feet (not flippers) and no collar.
Kermit is the most famous Muppet creation of puppeteer Jim Henson, who played the character until his demise in 1990. Steve Whitmire has performed this role ever since. Kermit has also appeared in various sketches on Sesame Street, in public announcements and commercials over the years.
Initially, Kermit was a lizard-like creature and it was not until he appeared severally on television that his status as a frog was established. This was a short time prior to the Sesame Street debut. And why does he wear a collar? This one was added during the early days to make him appear more frog-like, and also to cover up the seam linking his head and body.
“When I started doing this little local television show, Kermit was more a lizard-like character. We frogafied him over a couple of television specials we did years ago, before Sesame Street. So he just slowly became a frog. I don’t think there was a conscious move to do that…” Karen Falk, a Henson Archivist, quoted on the issue made by Jim Henson prior to his passing.