Have you ever seen one of those caper films where theft from the rightful owner takes place near the beginning and the gang spends the rest of the movie stealing the loot from each other? In the comic book industry, that's known as Tuesday.
For a beautiful example of the tangled web, take a look at this story from the B&W comic Psycho (itself a rip-off of Warren's Eerie). This would appear to be a rip-off of Marvel's Man-Thing until you check out the dates. The Heap story came out slightly before the original appearance of the Man-Thing which came out slightly before the original appearance of the Swamp Thing.
Comic book publishers spent much of the early Seventies looking for the next big thing (remember, the Golden Age wave of superheroes had come and gone in less than a decade). A number of publishers (particularly Marvel) bet heavily on horror. Is it possible that all three just happened to come up with the idea of a swamp monster at the same time?
Both Man-Thing and Swamp Thing were obvious imitations of a well known character from the Forties and Fifties who had spent more than a decade lumbering around the pages of the popular comic book, Airboy (Hillman comics), sometimes even having a cover to himself. That character was known as the Heap.
So, was this obscure magazine from the obscure publisher Skywald the only player here not involved in intellectual property theft?
The 'wald' in 'Skywald' was Israel Waldman, a notorious figure in the industry who had built a career on publishing things he didn't hold the copyrights to (Don Markstein has a good account of Waldman's business model here). Where the creators of Swamp Thing and Man-Thing lifted the broad outlines of the character, Waldman almost certainly stole him outright (not unlike the way another publisher approached Ghost Rider).
But we can at least credit Hillman Comics for coming up with the original concept back in the Golden Age, right?
The people at Hillman clearly got the idea from the original muck monster story, Theodore Sturgeon's classic short story, "It." Sturgeon, of course, never saw a cent of the profits from the Heap, Solomon Grundy, Man-Thing, or the Swamp Thing.
None of which should keep you from enjoying the fine Andru/Esposito art below.