Skaven or Reepicheep; you'll address them as "Skritt" and do it with respect if you know what's good for you.
In a new ArenaNet blog post, Ree Soesbee reveals the latest Guild Wars 2 monster to plague the land: the "horribly, wickedly tricksy" Skritt. Skritt are a humanoid blend of rats and bats that live in massive colonies under the surface of Tyria, emerging to mooch and presumably harvest spoons from their victims.
One interesting fact about this race is that Skritt share intelligence in an unknown way, and the more of them that congregate the smarter they become. It's this tenacity and hive-mind that have made them mortal enemies of the Asura, who are adamant that all Skritt be wiped out.
So what prompted the creation of Guild Wars 2's Skritt? "We hadn't explored the idea of a hive-mind in our world before, but we didn't want to just repeat the old standard-an insectoid race. We went through many ideas, and eventually settled on the concept of a rattus rattus based horde. A major consideration in their creation was that this race had to challenge the asura on their own terms, but not be just another super-intelligent species."