Ratbert

Ratbert is a regular character from the Dilbert comic strip. He was not originally intended to be a regular, instead being part of a series of strips featuring a lab scientist's cruel experiments (Ratbert's name at this stage was XP-39C²). Ratbert soon realized that he was the subject of a hideous macaroni and cheese experiment (the scientist made him eat huge amounts of it and writes in his notebook that it causes paranoia in rats) and escaped, eventually finding a refuge in Dilbert's house. He was not initially accepted by the residents, especially Dilbert, who was highly prejudiced against rats. However, he finally allowed Ratbert to become a permanent member of the household.