The Frog Prince was in 1971...however Kermit was a Frog before that. In Hey, Cinderella! (1970) he was a frog, and in the first season of Sesame Street (1969) he was a frog, in The Muppets On Puppets (1968) Kermit (then still flipper-less) was referred to as a frog two times and even before that he was known as a frog.
Here's a quote from Jim Henson (circa 1985) on the issue:
"When I started doing this little local television show, Kermit was more a lizard-like character. We frogafied him over a couple of televisions 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... He's very primitive as a puppet goes because he's really like a glorified sock puppet. But he changed a little bit - a few years after I first made him because when I first made him, he was - all my characters in those days were abstract. And he was sort of a lizard-like character. And then after a few years, we changed and made more of a frog-like body for him and gave him flippers for feet."
Why a frog? Why not a lizard, a toad or gecko? Well, I don't know...I don't know if anyone (even Jim Henson, himself) would know for sure.
