In Tales to Astonish #27 (Sept 1961), a one-shot quickie sci-fi / horror tale “The Man in the Ant Hill!” establishes the second major character in the fledgling Marvel universe: Hank Pym! Despite being mocked by the scientific community, Hank develops a serum that can shrink things. Okay. After trying it on a chair, he does what any good scientist would do next: try it on himself.
Woops! Now he’s the size of an ant. After a couple pages of expected peril, he manages to befriend an ant who helps him get back to his laboratory and restore his size.
I’m sure originally this was just a one-shot idea and they weren’t deliberately copying DC’s Atom who had debuted only a couple months before. But as Marvel decided to lean into super-heroes months down the line, I guess Stan Lee saw an opportunity to wrap up Hank in tights and turn him into one. Who knew he would become so prominent that he would have three blockbuster superhero movies featuring him?