Rats are one of those animals that have a complicated relationship with humanity. They're loving pets, but they are also at least partially to blame for the Black Death. I mean, technically the plague was passed by fleas, but it was the rats that acted as their public transportation.
They're adorable, but also pretty gross when you find them tearing apart your trash bags for food. Rats also procreate like crazy, which is part of why they're a popular lab animal. That, and people complain less when you experiment on rats than say, cute bunnies.