Ratfolk

I guess most folk in Novis Terram don't know this but there is an entire culture of ratfolk living under their streets. When the first elves settled on the coast there was already a thriving population of ratfolk and squirrelfolk living together on the land. They were basically kicked out of their homes and the squirrelfolk moved away but the ratfolk decided to move down.
Small and clever, the race has done fairly well for itself despite living in tunnels and away from the sunlight they had evolved to thrive under.