If so, then I'd like to know who it is...

No

@mr.obvious: I can only think of a FEW. HYDRA (Or A.I.M.) could be considered a hated foe because they tried to steal Ted Salis' formula and he was forced to become the Man-Thing just to keep it away from them! F.A. Schist cam close and after drinking water from the fountain of youth he fought a weakened Man-Thing and actually started WINNING! But when Man-Thing managed to get back to full strength again he was able to use his "Burning touch" and since the drinking the water from the fountain makes you immune to everything except being burned? It did NOT end well for him... The only other enemy I can think of for Man-Thing would be pollution, I don't think it could kill Man-Thing like Captain Planet, but it could make him sick, woozy, or just less potent on the battlefeild. So it wouldn't exactly HELP him either!

@infonation: Thank you for responding! :) Although A.I.M. was a major part of his origin, I never considered before that the organization as a whole could have a "thing" against Man-Thing (get it? :P) lol, also the idea of pollution would make sense, although NOT a living conscious being with a grudge against Manny, but it still causes him major damage! >.<

Mirrors

Btw, I knew already that Swamp Thing had his own archenemy (Anton Arcane), and I wanted to know if the other swamp creature Man-Thing had one as well...

F. A. Schist

After reading quite a bit on Manny the past few weeks and doing some online research, I've found that Man-Thing has three major enemies (although I personally wouldn't call them arch-enemies, per say):

Captain Fate - The ghostly commander of an ancient pirate ship has appeared quite a few times in the Man-Thing comics. He has formed a personal grudge against the swamp-creature and has taken him prisoner as well as tried to kill him many times.

Thog - Also known as the Netherspawn, powerful demon from another dimension who has fought Man-Thing and his allies Dakimh the Enchanter, Jennifer Kale, Korrek the Barbarian, and Howard the Duck.

F. A. Schist - A wealthy, greedy land developer who tried to tear down Man-Thing's swamp. Was killed by Manny but referenced later in many issues. Appeared in the Man-Thing movie.

D'Spayre has faced Man-Thing a handful of times, off the top of my head. I remember there was this nice twist where he made Man-Thing feel fear, which basically caused Man-Thing to start burning himself to death.

@Meteorite: Thanks! I'm a fan on Manny, and even I didn't know that!

No.

@infonation: I can only think of two cases where the two fought, though... Marvel Team-Up #69 (or thereabouts, it was part of Claremont's run), and Uncanny X-Men #144. I think.