Platypus is an egg-laying mammal.Mammal groupsMammals are divided into three groups - monotremes, marsupials, and placentals, all of which have fur, produce milk, and are warm-blooded.Placental mammals, like humans, whales, rodents, and bats, generally give birth to well-developed young.Marsupials-give birth to small, poorly developed young, and most female marsupials, such as kangaroos, wallabies, and the Koala, have pouches.Monotremes -are platypus and echidnas and the females lay soft-shelled eggs.