Unbirthday Week:

I turned 35, and decided I'd give YOU a present. Maybe more if there's time over my little holiday...

Goblin Patches and their perils:
The light of civilization doesn't reach far in a wild, hard-scrabble fantasy world. At the dim, grey edges of organized sapient culture, there exists a twilight realm of unconscious fear and anxiety where fears take living shape and begin to colonize.

Goblin Patches are an aggressive fungus that can begin anywhere the refuse of civilization can be found...sewers, garbage piles, latrines...whatever you like. These patches produce a mess of plant goblins (read +James Young 's idea below). Use your favorite stats, and number of appearing stats.

Older patches may produce goblins with specialized functions, and higher HD or wounds. Low HD or non Wild Card "goblins" either explode when taken down (+Jack Shear 's idea below) or have a 2 in 6 chance if you're feeling nice. Large patches might inhabit entire sections of wilderness or dungeon and co-opt other boring encounter-type monsters making them fun again...(giant ant with a mushroom growing from its head anyone?)

Possible weaknesses to exploit:
Salt (or other common thing like Vinegar)
Fire
Sunlight

Evidence of possible problem:
Missing pets (or other vulnerable targets)
goopy-exploded slime where exploratory goblins have died errupted
someone turns up with a gross patch of skin after being "bitten"
another type of low-power monster turns up, with fungus sprouting from skull...

Possible complications:
explod-y goblins are spore-spreaders helping the patch expand...
fungus is also magical and therefore can't be starved out without some kind of arcane component (it feeds on FEAR I tell you!)
growth has a rapid "infection" rate in certain monsters or terrains
it never really "dies" until you destroy the progenitor growth spore (queen, CPU, mother-brain what have you)

Foundations of this idea:
http://tenfootpolemic.blogspot.com/2013/05/plant-goblins-for-lotfp.html

http://talesofthegrotesqueanddungeonesque.blogspot.com/2012/06/mucus-goblin.html

http://en.wikipedia.org/wiki/Ophiocordyceps_unilateralis