Cinnamon is a guinea pig who was mentioned by Marge in I, (Annoyed Grunt)-Bot.
In the episode, after Julius Hibbert rides over Snowball II which kills the cat, Lisa buries Snowball II in the backyard. Marge tells Lisa about a guinea pig she once had as a little girl named Cinnamon. Marge then proceeds to remember the dead guinea pig and weeps "Oh! Cinnamon, meaning that Cinnamon died after it chewed through an extension cord.