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Britney Spears has appeared on Family Guy a few times.

Appeared in Family Guy Viewer Mail in which Peter disguises himself as her in order to steal Justin Timberlake's hair for Meg, they fell for the gag dispite Peter didn't attempt to disguise his voice.
She is mentioned in Model Misbehavior with Peter referring to Lois
She was seen at a buffet in Prick Up Your Ears with her dietitian. She was eating excessively and her extremely timid dietitian attempted (unsuccessfully) to keep her from eating.
In The Tan Aquatic with Steve Zissou, she was talking on the phone when someone rang the doorbell, she extinguished her cigarette on her baby's head, and dropped him.
She appeared in "Britney's Back", her comeback performance, in Family Guy Presents Stewie Griffin: The Untold Story!. She was severely overweight, had diabetes, and had a peg leg. She decided to try to dance in her wheelchair and fell on a radio and caught on fire.

Britney Spears