Back before they became heroes (and stars of their own title), the Squadron Supreme were villains. Appearing first in The Avengers #69 (October, 1969), they didn't even rate a cover appearance, but by issue 70, they were unveiled in all their glory (as seen on the right).
As envisioned by Roy Thomas (aided and abetted by Sal Buscema and Sam Grainger), the Squadron Sinister, as they were known, were an in-joke on Thomas's part, modeled after the then current membership of DC's Justice League of America.
To those out of the loop (and if memory serves me correctly), on the left of the cover we have Hyperion (Superman), Speed Demon (The Flash), Doctor Spectrum (Green Lantern) and Nighthawk (Batman). (I'll also parenthetically add that the big buy in the upper right is Goliath, but not the Hank Pym version. It is the Clint Barton (nee Hawkeye) verion...ain't trivia fun!).
Thomas had long been a fan of both the Justice Society from All Star Comics and their 60s counterparts, The Justice League (and had even parodied the JLA in his fanzine Alter Ego). With his interest in the JSA and JLA, it's no surprise he would want to bring the characters to Marvel in one form or another.
Proving that a good in-joke deserves to be repeated, Thomas revived the Squadron in The Avengers #85-86, but dubbed them the Squadron Supreme, making it clear that the other Squadron had been a group of evil doppelgangers and that this Squadron existed on an alternate earth (no, I am not making this up!).
After another appearance or two the Squadron disappeared into the land of Unused Supporting Characters (known to continuity buffs as Earth-Trademark) where it looked like they would fade into oblivion, but.... (continued in Part 2)
Kind of makes you go HUH?, don't it?