Dedicating myself with one of the most beloved Marvel monsters of all time, Man-thing, I decided to share the interesting story as to how the "Man-Thing and Swamp Thing Rip-Off" myth came to be and the possible true story behind the similar creations of the two famous Swamp monsters. 
Which Monster is a Rip-Off?

Biggest question for comic fans spotting the striking similarity between both monsters and the fact they both were introduced on the same year, the question of which monster is a rip-off begins rise. With Man-Thing being introduced in May 1971, Swamp Thing would make his introduction a month after in July 1971. What made more of an interesting discovery was how both monsters shared the exact same origins. So with both monsters being closely introduced into comics, one would predict an instant lawsuit between DC and Marvel for copyright infringement. Even with the fact that both artists for both monsters were rooming together when creating both of these characters, Marvel decided not to pursue legal action against DC.

Technically Swamp Thing would be a Rip-Off from Man-Thing despite how writer Len Wein claimed that he was unaware of the resemblance between both monsters. But what made this whole situation much more interesting and why Marvel decided not to pursue legal action was by the fact that both swamp monsters would closely resemble another monster that made his appearance in 1942.

Roy Thomas, Co-Creator of the Man-Thing was known to be a huge fan of the original Heap monster and desperately tried to have the monster revived. Of course Thomas never wanted to treat Man-Thing as a revived version of the Heap, even though the Marvel writer did favor his new swamp monster creation. Plus the idea for Man-Thing did originate along with Editor and Chief, Stan Lee, which of course both men neglected to care for the striking resemblance that Man-Thing had with the Heap.

With Gerry Conway co-writing Man-Thing’s first black and white story, Wein would write Man-Thing’s second issue where the monster would ally with Ka-Zar against AIM. It was during this time Wein was roommates with Conway and that Wein was also working on his own title for DC known as Swamp Thing. By the time Marvel realized that both Man-Thing and Swamp Thing held similar origins, Marvel at first planned to take legal action and decided to drop it, possibly because of how Man-Thing closely resembled the Heap and that that Man-Thing was also probably a copyright.

Of course most fans grow curious as to which monster is a rip-off, but the possible truth is that both of them would be a rip-off.