This would be a rat that would try to sneak up on the players.
If it can see the players looking at it, it disappears shortly after (like the gutter runner)
If it gets close enough to the player, instead of attacking, it steals an item. Perhaps potions, perhaps books or grims, perhaps even ammo.
Stolen items can be retrieved either by killing the pickpocket, or later in the sack of the loot rat(if any spawn)