40d:Thief

In Fortress mode, a thief is a disgusting dishonorable creature that can skulk around the map unseen until it moves adjacent to a dwarf, guard dog, or other helpful creature.

There are two types: kobold pilferers and goblin baby-snatchers:

Kobold thieves can start appearing quite early (first year) and are generally immune to any traps; they only trigger traps in panic after being detected. Armed with only a copper dagger they are no threat to anyone. While they sometimes steal low value items strewn outside, there seems to be a mechanism that makes them target other items inside instead, too. A successful theft will be announced once the thief leaves the map.

Goblin snatchers happily run into any kind of trap. Armed with an iron dagger they should be approached with a little bit more caution, but any average soldier should take them down unharmed. When they get their hands on one of your beloved(?) children, they will stuff them in a bag they carry with them for that purpose.

Both kinds will usually come in small groups of about 1 to 4 individuals. They try to escape once detected and only fight when cornered. Their ability to sneak into your fortress is not to be underestimated; they can lurk around the map for quite some time and wait for their opportunity, even when they are already inside your fortress. They may arrive more or less simultaneously with sieges or ambushes, distraction and general turmoil working in their favor. Guarding any entrance with chained dogs is a safe coutermeasure though.

Certain "playful" creatures will also try to steal items from your hoard. These include raccoons, rhesus macaques and mandrills(which aren't just playful, but downright dangerous). In addition, bears and gnomes will try to steal your booze if they can get at it.