Mammals were a class of animal. Mammals, or analogues to them, some sentient, appeared on Earth and many other planets.
Harry Sullivan identified the Zygons as mammals, as they depended upon the lactic fluid of the Skarasen. (TV: Terror of the Zygons)
Certain scanners searched for mammalian lifeforms. (TV: Earthshock)
Craving a cat to cook, the Sixth Doctor stated that "small mammals [were] 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 such as humans "all look[ed] 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)