Pigs are animals that are seen within the Mario series. Pigs in the series also tend to be either anthropomorphic and are able to speak in English.
In Paper Mario, Pigs in this game are seen as unique creatures called Li'l-Oinks. They live in the Li'l-Oink Farm in Toad Town. There are many different kinds of Li'l-Oinks and if a certain number of them are contained then release, then an item may be left behind.
Pigs make a somewhat large role in Paper Mario: The Thousand-Year Door during Chapter 4: For Pigs the Bell Tolls". During the events of that chapter, the citizens of Twilight Town known as Twilighters are being turned into Pigs by Doopliss. It should be noted that even non-Twilighters such as The Traveling Sisters Three and Dupree are even turned into Pigs during their visits in Twilight Town. After Doopliss is defeated, the Pigs turn back into their respective characters.