Lion

Lions were a species of carnivorous mammal native to Africa and Asia, Earth. The Doctor and his companions occasionally encountered them, usually captured ones in a zoo.

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. (NSA: The Slitheen Excursion)

On a visit to the Ngorogoro Crater in 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. (DWM: Prelude: Birthright)

The Third Doctor and the Brigadier 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. (TVC: The Arkwood Experiments)

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. (DWBIT: Flight and Fury)

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. (AG: Doctor Who and the Pescatons)

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

Ian Chesterton 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 Nero. (DW: 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. (AK9: 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 a forest close to them. (EDA: Legacy of the Daleks)

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. (NA: Blood Heat)

Cultural references to lions

 * The Lion, the Witch and the Wardrobe was a book by C. S. Lewis. (DWM: 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. (DW: Rise of the Cybermen)
 * Nelson's Column, a monument to Horatio Nelson, had four lion statues around it. (DW: The Dalek Invasion of Earth)