In its day, Swamp Thing was the equivalent of last year's 'Blair Witch Project'. It came from nowhere, debuting in DC's House of Secrets as an eight-page short, and went on to become enormously successful.
Len Wein and Berni Wrightson fashioned Swamp Thing as both a love story and a horror story. A monster emerges from the swamps to watch over and protect a young widow, whose husband died in a tragic laboratory explosion. The story ended on the revelation that the monster was in fact her husband, rejuvenated and reborn by the same swamps he had disappeared into following the explosion.
It's an old axiom in the publishing world that, if you stumble across something that proves successful, the next step is to exploit it mercilessly. Scant months later, the first issue of the new, ongoing Swamp Thing series appeared on the stands. The series lasted twenty-four issues, with Wein scripting only the first thirteen, and Wrightson drawing the first ten. With all due respect to the writers and artists who followed Wein and Wrightson, most Swamp Thing fans seem to enjoy pretending that those subsequent issues never really happened, and that they were just a publishing aberration.
Some people take it even further, feeling that the saga of the Swamp Thing flowed directly from Wein and Wrightson into Alan Moore's adoption of the character, ignoring not only the remainder of the original series, but also his guest shots alongside Deadman in Challengers of the Unknown.
(Digression: the final issues of the Challengers series are fun reading, by the way. Some nice sci-fi action scripts, coupled with some Kirby-esque pencils by Keith Giffen and some Adams-esque pencils by Mike Nasser, both of them then relative neophytes to the comics industry.)
Any talk about Swamp Thing's history wouldn't be complete without mentioning Marvel's own muck-encrusted monster, the Man-Thing. I'm not entirely clear on Man-Thing's history (I was never a Marvel zombie, after all), but I understand that some of the basic character hooks were the same. DC and Marvel created both characters independently and more or less simultaneously, from all reports. Just ridin' that synchronicity highway...
Dave Sim put together a nice parody of the situation in the pages of his 'Cerebus the Aardvark' series a while back, with 'Sump Thing' and 'Woman Thing' enjoying some, um, consensual, um, interaction, much to Cerebus' bemusement and Professor Charles X. Claremont's demise.
In most of the Swamp Thing stories between the Wein/Wrighton run and Alan Moore's take, the horror elements of the original concept largely fell by the wayside. Most writers used him primarily as muscle, sometimes using a swamp or bayou theme, leaving aside the influences of the earlier stories. All bets were off when Moore took over, though.
Moore reinvented Swamp Thing as a plant elemental, created to be a protector of the world's vegetation. The key difference introduced by Moore was that Swamp Thing had been mistaken in believing that he had once been human; rather, he was a pure creature of the swamp, distinct from the body and soul of the scientist he thought he had been.
Oh, stop snickering.
Moore's run on Swamp Thing was notable for several reasons. It sparked the whole Vertigo line, for one thing, and was the first mainstream comic to bypass the Comics Code Authority completely on a regular basis. It also introduced a new blond, spike-haired character named John Constantine into the DC Universe. Sting, sorry, Constantine led Swamp Thing around the continental U.S. for several issues, in an extended storyline called 'American Gothic', exposing the green guy to a variety of horrific situations. You could say that Swampy was wrapped around his finger...
Rick Veitch took over the writing chores on the Swamp Thing series after Moore left, and tore into a plot which had Swamp Thing bouncing around through time, meeting various historical figures and DC characters along the way. The plot proceeded neatly, allowing some exposure to several under-utilized characters, such as the Shining Knight of 'The Seven Soldiers of Victory' fame, until Veitch proposed that our green friend meet up with Jesus Christ.
As you might expect, there was much consternation. In the end, DC Prez Jenette Kahn spiked the story, sparking a long, ongoing debate over the depiction of religious figures in comics and whether DC was justified in their decision. Frankly, I never understood the furore; considering that Superboy and the Legion of Super-Heroes once met up with the likes of Hitler and Nero, it's not the least bit objectionable for Swamp Thing to break bread (or is that break tubers?) with JC. It's not like Veitch proposed making Swamp Thing into one of the disciples, right?
That being said, I completely agree with and support DC in their assertion that the decision was theirs to make, cries from the gallery over censorship notwithstanding. Their right to make that decision is distinct from any question of the relative propriety of the material in question. This was the case for Veitch's Swamp Thing story, and it holds true for Kyle Baker's Superman's Babysitter story. As much as I wish that the latter had received a much wider distribution than it eventually did (because I think it's really, really funny), I still support Kahn, Levitz, et al. in their right to decide whether to publish it.
For the record, I'd be saying that even if I didn't already have scans of the story.
Thanks to Rich Handley for catching my 23/11 slip; the original series ran 24 issues, with Wein writing the first thirteen issues.