Matt of No-sword has a typically informative post about two words for ‘frog’ in Japanese, kaeru and kawazu; he says “The common answer is that kawazu is the ‘old word’ that got replaced by the ‘new word’ kaeru, but this is a misconception. It’s really just another case of semantic overlap combined with poetic versus everyday register,” and proceeds to give a detailed and convincing explanation which you should read if you have any interest in the subject. But I came here to tell you about it because of the kicker:
Kawazu would probably have been forgotten by all but the specialists by now (much like tazu) if it weren’t for one thing: the Dark Side of the Moon of traditional Japanese poetry, that one haikai by Bashō that everyone knows…
古池や かはづ飛び込む 水の音
Furuike ya/ Kawazu tobikomu/ mizu no oto
Old pond/ Frog jumps in/ Sound of water
Bonus fact: Bashō was actually consciously playing with the kawazu tradition here by attributing the sound to the water rather than the frog. The frog’s implied silence, after centuries of naku kawazu, is a crucial part of the stillness that allows the sound of water to make its impact.
Isn’t that interesting? I’ve read that poem umpteen times without having the slightest inkling of this basic information!