There's a lot of ways for rats to be "wrong".
  1. The classic ROUS
  2. The perhaps-even-more-classic plague rat
  3. Unusually hungry
  4. Misshapen - extra legs, extra heads, extra tails
  5. Emit poisonous gas (except why wouldn't Vi & Krosp have smelled it?)
  6. Intelligent. The Sparks are about to be overrun by 10,000 copies of The Brain
  7. Combinations of the above