aka "Herr Doktor Hubert"
Hmmm... at first I was skeptical, since their writeup in Volo's Guide reminds me more of "fey zombie apocalypse" (their numbers increase exponentially as they convert more and more of their victims) than "long-lost civilization".I don't know if this helps, but I always felt Meenlocs were a great degenerate race. Perhaps pre-goblin, with a curse on them from aggravating the goblins when the goblin civilisation was ascending, and the meenloc civilisation was declining?
Still... maybe the first meenlock got started when goblins beseeched the fey courts of Thelanis for revenge against the invading humans, and got their wish... sort of. They spread rapidly across the land, consuming the humans and depopulating the entire Wayward Coast region. But then they were starved of food, though a few of them managed to enter hibernation and wait for the day when there would be enough food once more...
Such "meenlock mounds" might dot the landscape, for adventurers to stumble across.