And what kind of RPG would this be without giant rats? A terrible one, that's what!
Frizzil plans on having giant rats spawnable in every biome, including underwater and in space.
Rat monster spawners can be found in towers.
Rats are quite bulky and small. This allows them to pass through spaces only a single block high, and also makes them annoyingly difficult to hit them. Warriors can kill them easily, but Mages and Archers should have the appropriate tools for fighting them. When fighting them 1 on 1 it's not a big deal, But in groups, Defence (Armor and/or Dexterity) is very important. If you are a Ranger, you should either shoot explosive arrows at them, and risk damaging yourself in the meantime, or you should always have a club our sword on you. If you are a Mage, try to find an open area, and run backwards while shooting your best spells at them (I suggest using lightning, for its fire damage and large AOE). If you can't do that (if you're in a jungle biome or other constricted place) then find some water and shoot at them from underneath it.