Now you may be forgiven for thinking DC ripped off Marvel's idea, but that's just not so. The characters came out so close to each other, and given lead times it meant that the two comic book houses were working on remarkably similar characters at the same time - as if both sets of writers almost had the same idea. It gets even more curious when we learn that the writers of each series were rooming together while working on the characters - more of that later.
"Like any business rivals, comic book publishers have routinely "borrowed" popular characters and concepts from their competitors. However, there were a handful of times when publishers created characters that were so similar, yet appeared so close together, that one publisher copying the concept from the other was a virtually impossibility." Comic Coverage
It is DC's Swamp Thing which is today the most well known character, which is largely down to Alan Moore's run on the title in the early 80's, but both characters have their fans and both have suffered appalling movie versions.
2005's Man Thing holds a 20% rating on Rotten Tomatoes while the best of the Swamp Thing movies manages a little better at 60%, but neither of the characters have had much of an impact on movie fans.
Looking at the image of the two characters (right) it does make one wonder - on the surface Swamp Thing would appear to be a rip off of Man Thing, but there was a reason that Marvel didn't take legal action when the DC character came out only a couple of months after their character. Marvel did at first intend to take legal action but decided to drop it, since both characters owed so much to the Heap. Maybe both characters were in fact influenced by The Heap that first saw print in 1942.
"I was rooming with Gerry Conway (co-creator Man Thing) who wrote the first Man-Thing story. It was just independent creation. We were doing Swamp Thing and Gerry and I think Gray Morrow was doing Man-Thing. Neither of us knew the other was doing the same thing. The weirdest aspect is that I actually wrote the second Man-Thing story; the whole “Whatever knows fear burns at the Man-Thing’s touch”. In Gerry’s first story anything the Man-Thing touched burned. It was a protagonist who could never interact with anybody so I came up with the idea of fear." Swamp Thing creator, Len Wein
And so there we have it - the story of characters that were so similar, both in looks and origin, that readers used to get the two mixed up, but ahh well - that's comic books for you.