There are actually three types of mammals. They are monotremes, marsupials and placental mammals. Monotremes are the least common and are mammals that lay eggs. The most popular example is the platypus. Marsupials are mammals that grow their young in pouches. The kangaroo is an example of a marsupial. The last and most popular group are placental mammals. As the name suggests they grow their young in the womb and nutrients are fed to the developing fetus via the placenta.