This works both ways, compare the "Germania" by Tacitus, which was really about a fictional foil to contrast with the Julian dynasty, but became a historic document and origin story for a few centuries. (Sad to say, but humanists had a decent share in this.)

Related literature: Chrisopher B. Krebs, "A Most Dangerous Book. Tacitus' Germania from Roman Empire to the Third Reich", W.W. Norton & Company, NY, 2011.