Brownies are small nature spirits. They are related to the fae; and like all fairies, are mischievous and flippant in nature. They look like prepubescent boys; except dressed up in animal suits with open faces. They have the personality of children and will never grow up. While not especially dangerous, they can be nightmareish for any party traveling through the wilderness if angered; throwing stones or even luring wild animals to your camp at night- they are known to speak to animals, even your own, and getting them to run away from their owners. They aren't especially malicious either, and have short attention spans. Their first reaction is always the strongest; if they see someone in an even slightly funny hat carrying a broom, they'll assume that must be a witch and will start to hassle you. Meanwhile if you're wearing a dress and give them candy when you first meet them, they'll think you must be some kind of princess and will come running whenever you cross their territory.
Brownies don't have stats. Just assume you can't catch them. If you do
manage to get into a fight with them, they can do 1 point of nonlethal
damage to you (sticks and bites) and you kill them in one hit. They're
basically just kids with just a sprinkle of magic.
Traveling in bands, Brownies collect all manner of useful items and secrets. Their valuation of things is based more on a gut reaction or how useful whatever treasure it is would be to a 8 year old. They'll gladly trade dirty and dingy gold coins for a nice set of ivory dice, or a handful of copper coins if the emperor on it has a funny face. Be careful if they have something really useful though, as if they sense you really want it they'll start demanding something absurd to trade, and may just refuse to give it up just so you can't have it in a sort of childish flex.
They have a strange fascination with hounds, and will lure away hunting dogs from their pack. This isn't out of spite, they simply don't have any "cute doggies" out here. It is also said that if you manage to catch a Brownie, tear them out of their suit, and force them to take a bath; they'll become a regular if somewhat unruly boy and grow into a normal adult- all the many countless years as a nature spirit fading from their memory like waking from a dream. Good luck catching one though as they can move through any type of terrain both without leaving any tracks and without being slowed down; passing through thick foliage and bramble as though it was an open field.
Finally; Brownies appear in animal suits based on where they are. In the European bland human kingdom, they'll appear as badgers, squirrels, hares, and bobcats. If you are in the not!Africa part of the campaign setting, they'll appear as warthogs, meerkats, baboons, and civets. It is also said that at a distance it is impossible to tell if they are a brownie or if they are the animal they dress up as; so seeing strange groups of different animals walking together is said to be a sure sign you are looking at a Brownie band.