According to his bio at Candi Comics, from which Menjou is on loan:
Menjou was a campus lab ferret. Over the years, through various experiments he became telepathic and a school prize. However, he wanted his freedom to be a normal, obnoxious, cute, talking ferret. So in exchange for his freedom he made a deal with the Squirrel Mafia. The problem was, after he was freed, he didn't want to participate in what the Squirrels wanted him to do and fled. After weeks of hiding from them and almost getting caught, he snuck into Linda's backpack and luckily found a safe new home (since the Squirrels are scared of "The Indoors"). He's pretty much under house arrest though, since the Squirrel Mafia is always watching...
As a result of Jessica's interference, Menjou has temporarily escaped house arrest and has been transported to several other worlds. Luckily, he can fly, talk, light up dark areas and look cute in his Kim Possible outfit, so he'll probably manage.