Man-Thing was created when scientist, Ted Sallis, injected himself with a failed ‘super-soldier’ serum (he had been trying to recreate the Captain America formula) and crashed into a swamp (later revealed to be a magical nexus) while on the run. The combination of chemicals, magic and swamp transformed him into the Man-Thing. Appearing about a year and a half before DC’s Swamp-Thing the origin stories are incredibly similar and apparently almost led to a legal battle between Marvel and DC. The characters diverged quite drastically after the origins though, so it never really came to anything. He’s had quite a few series over time and has appeared regularly throughout Marvel’s various titles since his creation and he’s currently a regular in the Thunderbolts title.
On a side-note, if you get a chance to watch the movie version – DON’T – it went straight to DVD for a good reason, absolute rubbish – you have been warned.