The link to the online Oxford in anonsi's original post is solid support for adding ribbit.
But the Oxford entry goes further and lists the word as a verb, suggesting that we should accept ribbited and ribbiting as well as ribbit. And these forms of the word are in use. For example, a Guardian headline in 2017: "Ribbiting stuff: museum app gives people chance to help in frog research". In the article: "Australia has 240 named native species of frog, but the museum wants to identify what it believes are dozens more still ribbiting under the radar."
So ribbit, ribbited and ribbiting will be allowed in future as rare - but hopefully not endangered - words.
