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.

Biology
The Twelfth Doctor told Bill Potts that her bacon sandwich once had a "mummy and a daddy", indicating that pigs reproduced sexually. (TV: World Enough and Time)

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)

The Ninth Doctor claimed to Rose Tyler that worlds had been invaded because of the lack of bacon on other planets. When Rose pondered how bacon could be worth fighting for, the Doctor told her to ask a pig. (AUDIO: Retail Therapy)

Two suckling pigs were among the food eaten by Shockeye and the Second Doctor, partially converted into an Androgum, on Las Cadenas, a Spanish restaurant, during the year 1987. (TV: The Two Doctors)

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
According to the Legend of Sutekh, the pig was among the animals associated with the titular Osiran god of evil and violence, Sutekh. Other animals associated by the Egyptian tradition with the cult of Sutekh included crocodiles, donkeys and hippopotami. (PROSE: Background)

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)

In 1857, many sepoys came to believe that their gun cartridges were being coated in pork or beef fat. As Muslims couldn't touch pork and Hindus couldn't handle beef, they mutinied. (PROSE: All-Consuming Fire)

The Eleventh Doctor once had to deal with some "evil pigs in blankets" one Christmas. (COMIC: Who Who Who, Merry Christmas)

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)