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