[one who swells the checks]. King of the Frogs, and son of Pelus [mud], slain by Troxartas, the Mouse-king.
Great Physignathos I, from Peleus' race, Begot in fair Hydromede's embrace. Where, by the nuptial bank that paints his side, The swift Eridanus delights to glide.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Physignathos from Infoplease: