Animal

An animal was a kind of lifeform. Mammals were a class of animal.

Trends
As noted by the Third Doctor, all animals had brains of some sort. According to Butler, overcrowding in mankind, as in all other animal species, increased hostility and aggression, which led to war, "the greatest crime of all". (TV: Invasion of the Dinosaurs)

On Earth
As a result of the plague which was unleashed upon the planet Earth by the Daleks prior to the 22nd century invasion, a lot of animals escaped from zoos. Though most were killed, reptiles such as alligators thrived in the sewers of London. (TV: The Dalek Invasion of Earth)

Around 10,000,000 AD, the Earth was in danger of falling into the Sun. The entire human, Monoid, animal and plant population of the planet was loaded onto the Ark, a gigantic spacecraft, to take them to a new home on Refusis II. (TV: The Ark)

Elsewhere
During the Thousand Year War on Skaro, the planet's animal life began to mutate at a rapidly accelerated rate due to the nuclear and chemical fallout of the war. (AUDIO: Innocence) Most of Skaro's fauna was later driven to extinction by the first generation of Daleks detonating a neutron bomb to end the fighting, (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) though some had survived in the Lake of Mutations, being used as a natural ward for the Dalek City. When the First Doctor and his companions arrived on Skaro, they encountered the petrified corpse of a magnedon, an animal composed of metal rather than flesh, blood and bone. (TV: The Daleks) Another surviving example of Skarosian fauna was the Slyther. When the Dalek Empire spread to other planets, they used Slythers as enforcers. (TV: The Dalek Invasion of Earth)

Proximans were a semi-intelligent, empathic telepathic species of social animals, with a type of primitive group mind described by the Eighth Doctor as a co-ordination. They lived in nests. (PROSE: The Face-Eater)

On Harmony, all animal life (or, at least, those raised for food) was wiped out in a plague. (PROSE: Planet of Bones)

Gallifrey once had a rich diversity of multi-coloured animals but the Last Great Time War had seen the extinction of all them but the flies. (PROSE: A Brief History of Time Lords)