What is the ring of triangles around Kermit's neck?
Looking at pictures of frogs, I have been unable to find one that has similar markings. For example, take this picture of a typical frog:
Kermit, of course, has a signature ring of triangles around his neck:
So far I have been unable to find any credible evidence of what that ring is. No interviews with Jim Henson, for example.
One reason given is that ring hides a seam between the puppet's head and body: while this gives a why it is there, especially for a puppet built in the 1950s without modern technology, it does not explain what it is. Another is that Kermit started out as a lizard or nameless animal and later turned into a frog: however, would his appearance not be updated to match his species? Again, this theory does not explain what it is.
This thread on Reddit touches on the various theories and is a good, concise overview: however, it offers no credible evidence to back up any of the claims of what that ring is.
Note: if you follow the Wikipedia links for Kermit there are some articles that assert he was species-less at first with Henson quotes to back it up: however, upon becoming a frog I would expect his features to match that of a frog as close as one could expect a fictional anthropomorphized muppet to match. His original species is largely irrelevant to his frog features.