In SlashTHEM, one brownie is generated asleep within the village of the Town branch.
Brownies are especially dangerous when awake; they are fast and possess spells such as destroy armor and curse items, on top of being able to steal items and teleport away. They also have many more hit dice than the typical nymph, although they only have one seduction attack.
While not a particularly damaging threat, they are annoying and generally not worth the risk of a cursed inventory, stolen items or even destroyed armor. It is well worth to check if that n is a brownie or a mountain nymph before engaging it - if you decide not to avoid them altogether using stealth or other methods, be prepared to get rid of them quickly. Brownies can be dispatched most safely using available ranged weapons and/or Elbereth - be wary that even with magic resistance, allowing the brownie to get within melee range may lead to them whisking the source of your resistance off your person.