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" as he was turning into an Androgum. (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)
Time Lords were described in the I-Spyder Book of Earth Creatures as a "bipedal, bicardial mammal". (PROSE: The Last Dodo)