Cuddles is a hamster that ate Monster Blood and grew to about ten feet. His only appearance to date is in Monster Blood II and is technically the villain of the book. He is one of the more popular characters in the series. Sometimes he's called "Cuddles the terrible hamster."
Cuddles is owned by science teacher Mr. Murphy, who punishes Evan Ross to clean Cuddles' cage, whom Evan dislikes. To get back at Mr. Murphy for Evan, Andy feeds Cuddles some Monster Blood. But after awhile, he grows to an enormous size and starts rampaging through the school. Evan eats some Monster Blood himself, so that he can grow to Cuddles' size and do battle with him. However, Cuddles is about to finish Evan off, until they both return to their normal sizes. It turns out that the expiration date on the Monster Blood has passed, resulting in the magic wearing off. As a present, Cuddles is given to Evan. In the end, it is heavily implied that Cuddles ate some more Monster Blood.
Despite only appearing in one book, Cuddles has grown to be a very popular character in the Goosebumps franchise. He's been used a lot in promotional material and merchandising as well.