The Monster Cookie is a mysterious character who appears in one of Cookie Monster's dreams. He is a personification of the phrase "You are what you eat." After Cookie Monster finishes some cookies his grandma gave him, he goes to sleep and has a dream in which he meets the Monster Cookie.
The Monster Cookie tells the sad tale of how he came to be: he was once a regular fuzzy monster, but eating only cookies every day caused him to turn into one. Now he regrets not eating healthier foods. Cookie Monster wakes up after his dream and decides not to eat cookies anymore. He eats some healthy foods (such as carrots, fish and whole wheat bread), but goes back to eating cookies afterwards, deciding "Maybe sometimes a cookie."