Pig

A pig was a four-legged animal native to Earth. They were farmed mainly for their meat, most of which was simply called pork, though bacon also came from pigs.

As food
Pork was a popular foodstuff on Earth, eaten by humans and other predators. It was also eaten by the Doctor. His second self once chose pork when confronted by a food machine on a rocket. (TV: The Wheel in Space) A pig's head with an apple in its mouth was one of the foods served to the sons of Grekk. (PROSE: The Sons of Grekk)

However, certain religions, like Islam and Judaism, forbade the eating of pig products, a practice that continued and evolved even after pigs became extinct. (PROSE: The Mary-Sue Extrusion)

As laboratory mammal
Pigs were used for a variety of experiments, likely due to their prevalence and biological features similar to humans. The Sixth Doctor and Peri met Toby the Sapient Pig in Ostend in 1913. (AUDIO: Year of the Pig)

In [1930, the Cult of Skaro turned the less intelligent people they captured into pig slaves, humanoids with pig-like features, to work for them. (TV: Daleks in Manhattan/Evolution of the Daleks)

A pig was augmented into a bipedal creature by the Slitheen family in 2006 as part of an alien invader hoax. It was then planted in a ship to act as a diversion from the family. (TV: Aliens of London)

In the 51st century Mr Sin, a cyborg with the brain of a pig, was created. (TV: The Talons of Weng-Chiang)

History of pigs on Earth
As explorers explored the Earth, they introduced pigs to areas they weren't seen in before. Introduced pigs played a part in the extinction of the Dodo. (PROSE: The Last Dodo)

Cultural references to pigs
English speakers sometimes used the phrase "when pigs fly" to indicate the certain impossibility of a thing. However, the Eleventh Doctor once declared that a number of planets were home to a variety of pig that could fly. (COMIC: The Only Good Dalek)

Sock Pig was a toy made from a sock in the form of a pig. It was brought to life by the Anima. (AUDIO: Sock Pig)