Bogs have already been mentionned. However, I would like to add the following. Most bogs are highly acidic (from a biological perspective). This results in a rather difficult environment to grow in (the reason evergreen trees are so stunted). There are some interesting things with bogs, though. It is usually there that you will find carnivorous plants (the real ones, not the fantasy ones). The plants use insects for mineral input (particularly phosphates, IIRC) which are unavailable because of the acidity. Also, decomposition is slowed down to a crawl as bacterial activity is reduced. Finally, one of the dangers of certain types of bogs is the sphagnum moss. Although it has the wonderfull property of being able to suck up incredible amounts of water, it can sometimes create ledges that seem to be solid over open water. Once weight is put on it, it gives way and the individual sinks into the bog. Now in mid-summer that might be a bit disgusting but uneventful, but sink into water in mid-fall and try to keep warm after that !
Suggestions for creatures :
Giant carnivourous plants, giant insects, marshy creatures (goblinoids, reptile-men, etc), fleshy undead (zombies, ghouls, etc.), dragons.
Hope this helps.
Guillaume