I Can't Believe It's Not Butter! is a butter substitute produced by Becel/Flora/Promise, which is a subsidiary of Unilever. Advertisements featuring the product often end with the question, "I Can't Believe It's Not Butter, can you?"
Peter uses the I Can't Believe It's Not Butter! quote in "8 Simple Rules for Buying My Teenage Daughter".
In "Brian's Play", Brian replaces Peter's I Can't Believe It's Not Butter! with real butter, causing him to go on an insane killing spree that results in the deaths of three children.
After fighting a boxer named Butterbean in "Tales of Former Sports Glory", Peter then tackles another boxer named 'I Can't Believe It's Not Butterbean', but admits he couldn't tell the two apart.