Lion

Lions were a species of carnivorous mammal in the feline family. Natural prey for lions included zebra (PROSE: Birthright) — though they would certainly attack humans. (TV: The Romans) The Doctor indicated that, unlike other Terran animals (such as the horse), lions were unique to Earth. (AUDIO: The Bride of Peladon)

Encounters with real lions
The Doctor and his companions occasionally encountered lions, though typically specimens were caged in zoos or otherwise held in captivity.

The Doctor
When the Tenth Doctor and June visited Greece in the 16th century BC, they went to the Acropolis, where the Parthenon would later be situated. There, they were attacked by lions. The Doctor defeated one and they both escaped. Later, when they were leaving, they encountered the lions again, which the Doctor subdued by using his sonic screwdriver to tickle the beast. (PROSE: The Slitheen Excursion)

In about 120 AD, the Tenth Doctor found himself opposing lions, tigers and bears in a Roman amphitheatre. However, the lion was easily dispatched with the sonic screwdriver. One burst at the correct frequency immediately sent the cat to sleep. (PROSE: The Stone Rose)

On a visit to the Ngorogoro Crater in March 1926, the Seventh Doctor spotted some lions and was reminded of a time when he, perhaps in another incarnation, had seen the last lion on Earth shot in the same area. (PROSE: Prelude Birthright)

In New York City in the 1960s, lions were among the zoo animals released by a mad scientist. (COMIC: The Monsters from the Past)

The Third Doctor and Brigadier Lethbridge-Stewart met a lion at a zoo in the 20th century. The lion was as docile as an ordinary house cat. This odd behaviour convinced the Doctor and UNIT to investigate the strange goings-on in the area. (COMIC: The Arkwood Experiments)

In the 20th century, the Fourth Doctor and Sarah Jane Smith went to the London Zoo on the trail of a dying Pescaton. They noted that even the lions were afraid of the shark-like creature. (AUDIO: Doctor Who and the Pescatons)

In the 22nd century, the Tenth Doctor again crossed paths with lions, this time at a zoo. After a Dalek invaded the facility, he was momentarily surrounded by lions who had escaped. He escaped the pride only with the help of a vulgle. (COMIC: Carnage Zoo, Flight and Fury)

At some point, the Tenth Doctor and Martha Jones encountered lions in the wild in Kenya, whereupon the Doctor falsely advised that they'd be safe because "the lions were more scared of them" than they of the lions. (PROSE: The Pirate Loop)

Companions
Lions were also occasionally seen by companions who were separated from the Doctor.

Ian Chesterton and Barbara Wright were attacked by a lion near Rome. Ian managed to kill it, though he regretted it. (PROSE: Romans Cutaway)

A month later, in July 64, Ian saw a lion while imprisoned in the city of Rome. It was intended that he fight the beast, but this event was cancelled in favour of a duel for Roman Emperor Nero. (TV: The Romans)

K9 once encountered lions on an unnamed planet in another dimension, partially inhabited by ex-Tellac miners. They lived alongside other Earth animals from widely varying periods of evolutionary history. On this planet, prides of lions lived around acacia trees, males in the branches, females in the shade underneath. (PROSE: K9 and the Missing Planet)

Others
At some point after the 22nd century Dalek invasion, lions began to live in the forests of England. Men from Becca's village killed and skinned a lion from a forest close to them. (PROSE: Legacy of the Daleks)

In circa 10,000,000, a lion was one of the animals on board the Ark. (PROSE: The Ark)

Alternate realities
In an alternate timeline, lions survived despite the Silurians' reintroduction of extinct species. In Africa, Baryonyx would kill and feed on gazelle and leave the scraps for the lions. (PROSE: Blood Heat)

Cultural references to lions

 * The Lion, the Witch and the Wardrobe was a book by C. S. Lewis. (COMIC: The Professor, the Queen and the Bookshop)
 * In Pete's World, Mr Crane had "track 90" played once a large-scale cyber-conversion was underway in London. This song referred to sleeping lions. (TV: Rise of the Cybermen)
 * Nelson's Column, a monument to Horatio Nelson, had four lion statues around it. (TV: The Dalek Invasion of Earth)