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.

In 3300 BC, a wounded sheep entered the TARDIS, 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) According to one account, which may or may not 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) Much later, the Eleventh Doctor went in sheep dip and tried to get sense out of a sheep. (NSA: The Glamour Chase)

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

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.