As the creation of characters like Mister Fantastic and Elongated Man implies, heroes with elastic powers are clearly a favorite of comic book writers and artists alike, which also begs the question of “which one of them came first?”
Elongated Man was created in 1960, and Mister Fantastic would soon follow not but one year later with the creation of the Fantastic Four in 1961. Neither of them are nearly as old as Plastic Man, though, who was created in 1941 by Jack Cole.
Back then, Cole had been working for the aptly named publication “Quality Comics” - where the owner, Everett Arnold, had asked him to create a new character that resembled the hero the Spirit, for the planned “Police Comics” line.
Obviously, this didn’t happen but in hindsight, it was probably for the best since Plastic Man would later become a hit among readers and even star in his own self-titled series - thus, becoming a major earner for “Quality Comics.”