In Volo's Guide to Monsters, there is a race in the bestiary called Grungs. Were these created for 5e specifically?
I'm pretty sure they were created for 5e, as they have no deities listed which is always a bit suspicious, but I'm just making sure.
The grung are a Greyhawk-based amphibian race. They first appeared in Greyhawk Adventures (1988), then in Monstrous Compendium Greyhawk Appendix, MC5 (1990).
"Bath Time for the Hopping Prophet" is an adventure card from the City of Greyhawk boxed set featuring a cultist of Wastri and some grung.
They appear on page 27 of the Greyhawk Adventures supplement published by TSR in 1988. (Avalibable now on DMsguild) However, no dieties are listed for them there either.