From the Super Mario Wiki
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Pigs are animals that have appear throughout the Mario series, but unlike their real world counterparts, Pigs are often shown to be anthropomorphic or capable of speech.
In Paper Mario: The Thousand-Year Door, Doopliss turns the citizens of Twilight Town into Pigs with the ring of a bell. When Doopliss is defeated, everyone is turned back. The title of the chapter in which these events take place is called "For Pigs the Bell Tolls", which is a reference to the novel written by Ernest Hemingway called For Whom the Bell Tolls.