First Chapter Edit
As a monster appearing in Rolent's sewers, it's part of the game's combat tutorial section and it's the first enemy the player gets to fight.
Two variations of the creature exist with only a minimal difference in stats. The most prominent difference is that the first variation, only present in the first battle of the game, doesn't actually attack the player and merely stands in place until the characters kill it. The second variation, present in the last three battles, does attack.
Being part of the tutorial, the Dirty Rat isn't a particularly formidable foe, its only attack being a bite that does moderate damage from up close. The only time it can spell trouble is when all opponents in the battle focus on a single character.