When was the last time a character caught a cold or flu? Real people get them every now and again even with solid heating and dry interiors. Your characters are out in the cold and wet, and even inside is not always dry and warm. Imagine having to stop your dungeon crawl, sitting in a tight defensive position, while you all are feverish.
Arkths or Ravagers: These are large landbound crablike creatures about the size of Grizzly Bears. They live partially in the astreal plane. Their food is magic. They can sense it at huge distances. Since spells and such are hard to eat (being in motion) and magical places are the equivalent of a sea of krill to them, they are looking for magic stashed in concrete places. Their primary food source is magic items, alchemical potions, and enchantments. They do a pretty good job of mangling/ eating the physical aspect of said items in the process of chowing down. These things will be the bane of dungeons everywhere, as they will slowly eat away much of the treasure (and the magical traps... so it can be advantagous to follow these things). Note: They will attack people with highly magical natures (i.e. high magic powers). They will probably kill the person in the process of sucking out the power.
Khor are a monsterous and violent race. Within five years of being born, they grow to about half human size and are very, very inteligent. As they get older, they get larger... unfortunately they become less intelligent. After their adolecesence (about seven years), they grow to human size and are of a low human intelligence. As they age, they grow larger, slower, and stupider. Most of them end their lives 3 to 4 meters in height. Khor have been reported upto 10 meters tall, though those elder giant Khor do little but hit anything that disturbs them.
As they age they get larger, they don't seem to ever die of natural causes.
After particularly intense storm, a green glowing fungi can be found on the trees and plants. Also new monsters seem to be in the region. These storm are obviously bringing these things from other places and depositing them with the wind, rain, and goop upon the land.
An insidious creature, most likely somehow "related" to trappers and lurkers, the Dead Leaves (for no other name exists as of yet for this foul thing), hibernates for three of the four year's seasons, deep underground. Its active time is Autumn, when trees shed their leaves, depositing colorful carpets across the ground. The terror then emerges and blends in with the surrounding leaves, perfectly camouflaged, waiting patiently for unsuspecting victims. In appearance it resembles nothing more than a ten foot square, six inch thick, layer of bright yellow, orange, and red leaves. The only hint that someone is walking on top of it, comes in the form of an unusual amplified sound of leaves crunching underfoot. Too late usually, the victims notice this additional "crunch". The Dead Leaves will then swirl and "rise" up to smother and suffocate the victim, like a colorful, malevolent, boa constrictor.
Fire, as can be imagined, is particularly effective against this creature, but one has to *know* it's there before putting it to the torch. And there's the rub. The creature is impossible to "identify" in a large patch of fallen leaves by eyesight alone.