The frog/mouse connection dates back to Aesop, though his fable goes in different directions. About a generation before Ravenscroft it had been adapted by Robert Henryson, who gave it an interpretation in terms of Orphic mysticism (the frog is the body, the mouse is the soul, and both are swallowed up while tied together in the river of life by a hawk signifying Death).

Ravenscroft's story ends badly for all concerned but in a different way from Aesop/Henryson, and doesn't make for such a simple allegory (having the soul and body swallowed up by different supernatural forces would be like Rudolf Steiner's myth of the Eighth Sphere). So maybe it's a parody? If so did Ravenscroft make it up, or was it circulating before his time, and if it was, what on earth was its intention? As a kind of contest of rival myths, it's a bit like the processes Robert Graves imagines in The White Goddess, where proto-Christian monotheists tell parodic reinterpretations of pagan traditions.