The Swamp Thing character was created by Len Wein and Bernie Wrightson in the House of Secrets horror anthology for DC Comics back in the 70's. Therefore, "Swamp Thing" first appeared in House of Secrets #92 (July 1971) but that character is not the same as the one who became an established character in comics. The original Swamp Thing was Alex Olsen, but when Swamp Thing was deemed a hit the character was updated by his creators and became Alec Holland who first appeared in Swamp Thing #1 (November 1972). The Swamp Thing was released just a few months after Marvel released there own swamp monster called the Man-Thing.
Starring in his first ongoing series, which lasted for four years and twenty-four issues, a lot of Swamp Thing's cast of characters were introduced including mainstays like Abigail Arcane, Anton Arcane and Matthew Cable. However, it was until the second series (originally entitled "The Saga of the Swamp Thing") that Swamp Thing found the writer that would shape him in a run that is still one of the most acclaimed in comic history. With Swamp Thing #20, Alan Moore came onto the series and broke Swamp Thing down entirely and began to build him back from the ground up and make him a much more formidable being...transforming him from an almost-mute wandering lost soul to a plant elemental with the power to regenerate and travel the cosmos.
Alan Moore stayed on the book until #64, and was followed by several other writers such as Rick Veitch, Doug Wheeler, Nancy A. Collins (during whose run the title became part of the Vertigo imprint with #129) and Mark Millar but none gained the success of Moore and the title ended at #171 in 1996. It wasn't until 2004 that Alec Holland returned as the star of his own series (the third Swamp Thing volume, written by Brian K. Vaughan, featured his daughter instead) which was kicked off by a six-issue arc from Andy Diggle until Joshua Dysart would become the main writer up until the book's cancellation at #29. It was rumored in 2010 that China Mieville had proposed a fifth Swamp Thing volume but was denied because DC was no longer printing new comics featuring their properties through Vertigo. Alec returned to the DCU in Brightest Day #23.