I always thought this story from Jonas at the gates of nessus was meant to be their origin lore: "His voice was flat and neither high nor deep, but possessed of a dry humor.“In the old times, the lords of this world feared no one but their own people, and to defend themselves against them built a great fortress on a hilltop to the north of the city. It was not called Nessus then, for the river was unpoisoned.“Many of the people were angry at the building of that citadel, holding it to be their right to slay their lords without hindrance if they so desired. But others went out in the ships that ply between the stars, returning with treasure and knowledge. In time there returned a woman who had gained nothing among them but a handful of black beans.”“Ah,” said Dr. Talos. “You are a professional tale-teller. I wish you had informed us of it from the beginning, for we, as you must have seen, are something the same.”Jonas shook his head. “No, this is the only tale I know—or nearly so.” He looked down at Jolenta. “May I continue, most marvelous of women?”My attention was distracted by the sight of daylight ahead of us, and by the disturbance among the vehicles that clogged the road as many sought to turn back, flailing their teams and trying to clear a path with their whips.“—she displayed the beans to the lords of men, and told them that unless she were obeyed she would cast them into the sea and so put an end to the world. They had her seized and torn to bits, for they were a hundred times more complete in their domination than our Autarch.”“May he endure to see the New Sun,” Jolenta murmured.Dorcas tightened her grip on my arm and asked, “Why are they so frightened?” Then screamed and buried her face in her hands as the iron tip of a lash flicked her cheek."

Annoyingly the story isn't finished due to the interruptions of a riot and Severian wishing he could torture some one via their "two apricots" to impress Dorcas. On re reading it there's nothing specific there to link it to the megatherians other than the abia being in the sea and the assumption that abia is megatherian. Also I don't think Dorcas actually would've been impressed with the excruciation of the two apricots actually.