One of the oddest Real American villains is Great Warrior, a supernatural chief who killed himself and thereafter haunted Plastic Man in POLICE COMICS #16 (1943).
The story's curious animus toward Native Americans may be slightly mitigated by the fact that America was at war, and aboriginals were perhaps viewed by some as an "enemy within." I discussed the story in greater length here.
It's somewhat noteworthy that Plastic Man does not defeat Great Warrior's menace, for the villain easily possesses the hero and gets him in dutch with the cops. The hero's perpetual sidekick Woozy Winks is the one who saves the day, by getting Great Warrior's son to exorcise his old man-- even if it is one of the all-time "cheats" in the history of comics.