Sheep

Sheep were quadruped mammals found on Earth and domesticated by humans. They were raised mainly for food and wool.

Earth sheep
The Doctor, his companions and acquaintances occasionally encountered sheep.

At least one sheep actually came to inhabit the TARDIS — however briefly. In 3300 BC, a wounded sheep entered the ship, where it gave birth to its lamb. The Second Doctor left to find the sheep's owner, but when he returned he found the sheep and lamb brutally slaughtered. (DWY: Loop the Loup)

Some companions had at least some level of knowledge about sheep farming. According to one account, which may have been a dream, Jamie McCrimmon stopped travelling with the Second Doctor to become a shepherd in Yorkshire after Zoe Heriot went into a coma. (BE: Dream a Little Dream For Me) Dogs were integral to shepherding. In fact, Peri Brown once noticed a herd of sheep running down a dog on the island of Dorsill, which made her think that something was amiss on the island. (PDA: Grave Matter)

The disappearance or death of sheep was often an indicator of much bigger trouble. For instance, the Seventh Doctor and Ace initially went to the town around the Graystairs Alzheimer's disease treatment centre to investigate sheep disappearances. (PDA: Relative Dementias) On another occasion, in the Welsh village of Llanfer Ceirog, they saw a sheep being taken by a wolf. (NA: Cat's Cradle: Witch Mark). Similarly, in 1936, Harry Sullivan was falsely accused of being a werewolf who attacked an English village's sheep population. (PDA: Wolfsbane) In the 21st century, Ianto Jones and Jack Harkness encountered a dead sheep close to the village of Monknash. It was remarkable because it was completely devoid of insects. Instead it was perfectly undisturbed by any after-death predators. Having broken free of the normal cycle of death and decay, the sheep was a clear sign of problems yet to be uncovered in Wales. (TWA: The Sin Eaters)

In 2050, K9 Mark 2 briefly saw a sheep in the dream he shared with Alistair Gryffen. (K9TV: Dream-Eaters)

By-products
Sheep were obviously raised in part for their meat. Londoners were definitely known to be eating sheep meat as early as 1814, when Vicki Pallister ran across a mutton vendor in the streets. (CC: Frostfire)

Areas of farms dedicated to collecting sheep waste were called sheep dips. Distracted by discussions he was having with Rory, the Eleventh Doctor twice landed in a sheep dip that was close to where the TARDIS had landed. (NSA: The Glamour Chase)

Extinction
According to one account, sheep became extinct around 2106 after the Ozone Purge, though records of their existence survived. (NA: Lucifer Rising) However, this extinction was likely reversed on Earth, since sheep were known to have survived in human colonies off Earth.

Extra-terrestrial sheep
The colonists to Metebelis III brought sheep with them. They survived the crash. They later became a food source for the colonists and Eight Legs, though the Eight Legs preferred human meat. (DW: Planet of the Spiders) In the far future on Kirith, the Panjistri kept sheep on Kandasi Island as their main source of food. (NA: Timewyrm: Apocalypse)

Behind the scenes

 * In the dream world in K9TV: Dream-Eaters, K9 encounters a sheep, which his scanner verified as being 'electric'. This is a nod towards the Phillip K. Dick novel Do Androids Dream of Electric Sheep?.
 * The reference to Jamie becoming a sheep farmer in Yorkshire in Dream a Little Dream For Me is a metatextual reference to Frazer Hines long-running stint on.