Read Jeffrey first. And over on twitter, he's compiled these responses, from here and elsewhere: this, this, this, and this.
|Colmar, Bib. municipale 0409 (493), f.1r|
The tale of the mouse and the frog appears in all the major medieval British Fable collections: Marie de France (37-39), Berechiah ha-Nakdan (9-11), Walter of England (29-30), John Lydgate (358-525), Robert Henryson (2777-2975), William Caxton (one example). It's a fable worth remembering as we hear calls for compromise (eg: cf this to this).
In the fable, a mouse needs to cross a body of water, and, being unable to swim, agrees to let itself be tied to a frog; partway across the water, the frog tries to drown the mouse, and a bird of prey, hearing the struggle, snatches up the pair and has a meal. The moral is that tricksters will be tricked themselves.
An alternate moral, not one I've encountered in the medieval tradition:don't get on that frog's back.
A bird might rescue you and eat only the frog (Marie, Berechiah, Lydgate). Or it might eat you both (Caxton, Henryson). Maybe you'll get away, maybe you won't. Better just not to get on the frog's back.
(image from a manuscript [Colmar Bibliothèque municipale, 0409 (493)] of the translation into Latin by Carlo Marsuppini (1399–1453) of a classical Greek parodic epic, the Batrachomyomachia, The Battle of the Frogs and Rats. In this particular case, the frog drowns his passenger (who, in Chapman's translation, "Cried Peepe, and perish'd") because he's frightened by a water snake. War follows.)