A type of animal that has hair, bears live young, is warm blooded and the female has mammary glands that are used to feed the young, and give this type of animal their name. Some scientists believe they evolved from synapsids, who were more prevalent during the early triassic period before an extinction event opened things up so the archosauria could become more dominant. There are some types of mammals called monotremes who lay eggs and are thought to be earlier on the mammalian evolution branch. There are also the marsupials, of which the famous kangaroo and koala, are examples. These are non placental mammals, with the majority being placental mammals.
Mammals have appeared in The Land Before Time. One such example is Tickles and his family, from The Land Before Time IV: Journey through the Mists. Another is Skip who was in the TV episode Return to Hanging Rock. Milo, Lydia and Plower were three gopher like mammals who lived underground and are seen in the TV episode Stranger From the Mysterious Above. There was also one shown in The Land Before Time XIV: Journey of the Brave.