The 1940s Airboy comics included a backup feature called the Heap. The Heap was a World War I flying ace (for the Germans) who had been shot down over a swamp and came back to life as some odd sort of swamp monster/hero. It was an oddity about the Heap that he seemed to do the right (heroic) thing without apparently understanding the real concepts of right and wrong. He lasted for years in the Airboy series and even bumped the title character off the front page a few times near the end of the run.
In the 1970s, both Marvel and DC would have success with comics featuring knockoffs of the Heap. Marvel came up with Man-Thing, while DC was more direct with Swamp Thing.
Now, you can make a strong argument that the Silver Age featured a very similar (but not muckish) character: the Incredible Hulk. Okay, he didn't arise out of a bog, but aside from that he very much fits the type. And if you don't buy him as the Heap II, how about this guy who debuted a couple years before the Man or Swamp Things:
This story, of course, establishes Roy Thomas as the guy who brought the Muck back into Muck Monsters. It's got a swamp (in Florida, no less), it's got radioactive waste turned into a Superfund site thanks to the Hulk:
Well, that interacts with something below the surface and boy howdy:
What a terrific, dynamic picture and what a clunky name. Granted, Marvel was the company that unleashed Shagg and Rro and Spragg and Rorgg on the world in the early 1960s, and in fact featured another monster called Glob in Journey into Mystery #77 (September 1961).
Here's the origin of this Glob as drawn by Trimpe (who was really coming into his prime):
Some very nice sequences. And notice how Thomas steps back as narrator. Overall the story kind of fizzles out; the muck monster kidnaps Betty, and he and the Hulk have a little battle over that, but it turns out that the Glob didn't want to hurt her, and the water kind of dissolves him.