Thief

From DwarfFortressWiki

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. Some thieves can bypass traps as well.

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

Certain "playful" creatures will also try to steal items from your hoard. These include raccoons, rhesus macaques and mandrills. In addition, bears and gnomes will try to steal your booze if they can get at it.