Pig

Pigs are animals in the Real World and Mushroom World that have often appeared throughout the Mario series, but unlike their Real World counterparts Pigs are often shown to be anthropomorphic or capable of speech.

Super Mario Land 2: Six Golden Coins
They first appeared in Super Mario Land 2: Six Golden Coins as bosses known as The Three Little Pigheads, parodying the story of The Three Little Pigs. Super Mario Land 2 also featured a Pig sub-species known as the Cannon Pig.

Donkey Kong Land
In Donkey Kong Land, winged Pigs known as Hogwash appear as enemies and the game with their attack pattern mainly consisting of going back and forth in a set area.

Wario Land 4
A strange, hog-like creature named Hoggus appears in Wario Land 4; Hoggus could even make Pigs appear by sketching them on his pad.

Paper Mario
In Paper Mario, a species of pig named Li'l Oinks were available for purchase at Toad Town, where they could be used to obtain certain items. Unlike actual pigs however, Li'l Oinks are born from eggs.

Paper Mario: The Thousand-Year Door
In Paper Mario: The Thousand-Year Door, which had Doopliss turning the citizens of Twilight Town into Pigs with the ring of a bell. Doopliss was defeated, and everybody was eventually turned back. Mr. Hoggle, an anthropomorphic Pig, also ironically ran a Hot Dog stand in Glitzville.

Super Paper Mario
In Super Paper Mario, there are several foes that resemble Pigs (such as Squiglet and Pigarithm).

Mario & Luigi: Bowser's Inside Story
In Mario & Luigi: Bowser's Inside Story, Fawful's second-in-command is a monstrous pig-like creature named Midbus who acts as Bowser's main rival throughout the game.

Donkey Kong Country Returns
Pigs appear in the upcoming game, Donkey Kong Country Returns, where they mark the checkpoint whenever a Kong passes them.