The Rat (in-game name Rotta) is a common enemy in the starting areas of the caves.
Doesn't pose much of a threat on their own, but they tend to often aggro gun enemies, which may cause problems, even if the player is invisible. For example, enemies may shoot explosives next to the rat. Very frequent in Snowy Depths. The More Love perk mitigates this.
It is, without a shadow of a doubt a lovely lil' creature.
Similar to the player-spawned Ruttorotta.