Cattle

Cattle were domesticated mammals native to Earth. The males were called bulls, and the females were called cows. (TV: Image of the Fendahl) Clyde Langer once succinctly said — while he was looking after the baby Sky Smith — that "Cows eat grass, and they make milk and they go moooo... [and] they also make steak." (TV: Sky) Indeed, Rose once suggested to the Ninth Doctor that being out of cow's milk was a part of the reason that they often ended up back on Earth. "All the species in the universe," the Doctor replied, "and it has to come out of a cow." (TV: The Empty Child)

Encounters with cows
The Doctor and his companions and acquaintances occasionally dealt with cattle.

The Doctor
After materialising in a field, the Second Doctor and companions turned on the viewscreen to see the face of a curious cow staring at them. (TV: The Invasion)

The Fourth Doctor once landed the TARDIS amongst a herd of cattle and facetiously asked them (calling them "ladies") if they had a time scanner, to no avail. (TV: Image of the Fendahl)

The TARDIS materialised in the middle of a herd of cattle again when the Seventh Doctor visited Llanfer Ceiriog. (PROSE: Cat's Cradle: Witch Mark)

The Eighth Doctor also liked to talk to cows. After Destrii and he had defeated the Cybermen, he landed the TARDIS in a pasture. He spoke to the cows, telling them what a great day it was to have saved the universe. Noticing he had lost his frock coat in the battle, Destrii suggested that he would look good in leather (she was wearing a black leather jacket). The Doctor replied that that Destrii was once again "charming the locals", indicating that leather came from cattle. (COMIC: The Flood)

On another occasion, the Eighth Doctor (who was then split into three bodies), Charley Pollard and C'rizz encountered cows in the Garden of Curiosities. This meant that cattle existed not only off-Earth, but in completely different dimensions. One of the three Eighth Doctors noted that cows could become dangerous if they herded together and stampeded. He seemed particularly concerned about cow hooves during a stampede. (AUDIO: Caerdroia)

In 1913, the Sixth Doctor and Peri Brown witnessed the heavens open up to dump cows on the beaches of Ostend, Belgium. (AUDIO: Year of the Pig)

Companions
While briefly a vampire, Nyssa feasted on the blood of a cow in the fields outside Manchester in the 1990s. (PROSE: Goth Opera)

Bernice Summerfield once experienced a freak time anomaly in which leather shoes were turned back into cows. However, the time anomaly only restored the cows to shortly before they were turned into leather. The floors of the Gigamarket were littered with dead cows. (AUDIO: The Greatest Shop in the Galaxy)

Others
Dr Scotus, a veterinarian working near Cardiff, once examined a cow that had begun eating other cows because it had been affected by an alien tapeworm which had come through the rift. (PROSE: Slow Decay)

Even the Doctor's enemies used cows on occasion: Zygons once used body-print technology to take the form of cows, as they were allowed to roam freely. As was necessary for the proper operation of body-print technology, the Zygons stored the real cows in their ship. (PROSE: Sting of the Zygons)

When humanity became a space-faring race, they took cows with them to the stars. As Earth's influence spread, cattle were shipped across the galaxy, to planets such as Jadea (PROSE: Placebo Effect) and Sarath. (PROSE: City at World's End)