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.
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.