Lion

Lions were a species of mammal native to Earth. The Doctor and his companions occasionally encountered them.

The Doctor
When the Tenth Doctor and June visited Greece in the 16th century BC, they went to the place on which 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 Stone Rose)

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, saw 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 was only able to escape the pride with the help of a vulgle. (DWBIT: Flight and Fury)

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

For instance, Ian Chesterton saw a lion while imprisoned in the city of Rome. It was intended that he fight the beast but this event was changed in favour of a duel for Nero. (DW: The Romans)

Also, 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
In an alternate timeline, lions survived despite the Silurian's 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 4 lion statues around it. (DW: The Dalek Invasion of Earth)