Marsupials are a type of mammalian animal characterized by their unique reproductive process, where the young are born in an extremely immature state and continue developing outside of the womb, typically in a pouch on the mother's body. They can be found in various habitats, including the mountains, and some species exhibit diurnal behavior, meaning they are active during the day. This group includes well-known animals such as kangaroos, wallabies, and koalas.