Thumps hide in packs.
They live in forests and caves and some small towns. They're not only not frightened of fire, they're attracted to it. Thumps can lie dormant for a decade in spent ashes, waiting for hosts. Once could call them a pest or vermin but considering they don't eat or destroy anything and seem to grow on their own perhaps weed would be a better description.
In daylight, they're almost invisible, little flea-like things. In darkness, they only have physical form when they wish, and appear in the image of some kind of squid-tailed rabbit. They find a nice campfire and the pack goes to work. Like so:
They make noise to draw their hosts, be they of humanoid or animal intelligence, away from their place of rest and safety. They don't pounce upon them once isolated and take them down, though. The rest of the pack skitters in where the victim just left, filling their hay lofts or bedrolls or backpacks with their fluid forms, then poof-mostly vanishing. They're almost undetectable. But they're with you.
Thumps don't seem to eat anything, or have any waste, or breed as anything but little asexual clones. The next night some will sneak away from you in the darkness so that the litters they've had in the night can run free, making noises of movement, breaking trigs, clanging pans....
The longer you have a Thump problem, and it is hard to tell that you do, the less sleep and rest and peace of mind you have. The sound of invisible intruders everywhere - some say they even hiss like a whisper when happy - drives many hosts quickly mad. You'll start with some basic penalties to your rolls, then abilities, and then HP as exhaustion sets in...until you rid yourself of them or die gibbering. Then the Thumps hide in the dust, in the ash, in shadows, and wait for someone else they can sneak aboard.
There's no reason to think the little buggers aren't everywhere. They don't seem to want anything. There's also no reason to think they'd need to rattle and bang and distract and terrorize people the way they do: there's not much to stop them from coming and going when they please, releasing their spore litters, and just living apart from man and animal. Some theorize they do these things for fun, which makes them an exceedingly evil creature if true.
How one gets rid of Thumps is by burning all their possessions (Thumps are drawn to campfires and to dry ash but are not fireproof), or taking a long trip underwater, or having a Protection From Evil spell cast on you. You can never be sure you got them all, though. Some may still be under your toenails. Better take them off. Some may be in your mouth. Better kill em with hot oil. Some may live in your eyelashes. Take the torch to them...