While it's never been confirmed, just heavily implied; this cute little fuzzball seems to be the ghost of one of the guard dogs Axion Labs employed to guard the facility before Valerie's father and his high-tech security solution arrived on the scene. He first came back to find his beloved squeaky toy, and woe to anyone who got in the way! The chaos and destruction caused by his search ultimately cost Valerie's dad his job, their house, and her membership in the Casper High A-list. Unlike many ghosts in the show however, he's not really evil or malevolent: he just wants his squeaky!

He probably has some other name, however Danny nicknamed him "Cujo", and it seems to have stuck.