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

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)