Plenty of current Marvel characters existed BEFORE the Fantastic Four made their debut in 1961. Patsy Walker, Fin Fang Four, Captain America, Namor, etc.
However, Hank Pym made his debut a few months AFTER the Fantastic Four, but not as a superhero, but rather as a scientist who devised a way to shrink himself. This adventure later inspired Marvel POST-Fantastic Four to make Pym into a superhero (I guess they figured out that superheroes were selling, sorta like EC and romance comics!).
Reader, thekamisama, though, recalled reading a reprint of a Marvel horror comic that ALSO featured a scientist named Pym!
Was this the same guy?
Luckily, another reader, Hoosier X, solved this dilemma, where he fills us in that the reprint comic Tomb of Darkness #22 DID, in fact, reprint a story from Strange Tales #75 featuring the scientist thekamisama remembers, AND the scientist's name WAS Pym!
So it sounds good, no?
But as Hoosier X notes, this was an EDIT. The character's name was changed TO Pym, whcih was NOT the character's name in the original!
Tricky, Marvel, tricky!