In short, nations which never had chance of possessing a proper vexilllogy or heraldry, Coats of Arms attributed/proposed to some characters (like, how would a Western-style Coat of Arms of a Japanese Emperor looks like?), etc…
More here: https://en.wikipedia.org/wiki/Attributed_arms we call it "heraldyka fantazyjna" (fantastical/fantastic/fantasy? heraldry)
Presenting some of my favourites, Babylonia, Etruria, Romania/Roman Empire (two variants, one used OTL by reconstructionist group, but I like their Flag) and (Old)/Baltic Prussia (used by modern-day reconstructionists of this language on the terrain of Kaliningrad Oblast and around).
Also, if someone HAVE saved most of those unsourced/uncertain origin Flags deleted from Wikicommons in good resolution/svg (like those of "Turkic Empires", previous set of Mughal Empire and others) PLEASE post it here, I can't find a way to find them now.
Wikipedia used to have many good flags which are now deleted, whereas shit like combining random flag with homo flag stays, I can't comprehend this.