Mammals were a class of animal. Mammals, or analogues to them, some sentient, appeared on Earth and many other planets.
The Sixth Doctor stated that "small mammals are quite flavoursome when baked." (TV: The Two Doctors)
On Ravolox, Peri Brown sarcastically referred to "the rustling of small mammals". (TV: The Mysterious Planet)
Vastra claimed that mammals "all look alike." (TV: A Good Man Goes to War)