There are three types of mammals or groups of mammals that make up the main classification of these types of animals, believe it or not, including humans.
Mammals are vertebrate animals constituting the class Mammalia and characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, and three middle ear bones.Mammal – Wikipedia
The three types of mammals are the marsupials (pouched mammals), monotremes (egg-laying mammals) and placental mammals.
Marsupials or pouched mammals are mammals of an order whose members are born incompletely developed and are typically carried and suckled in a pouch on the mother’s belly.
Marsupials are found mainly in Australia and New Guinea, although three families, including the opossums, live in America.
Well-known marsupials include kangaroos, wallabies, koalas, possums, opossums, wombats, and Tasmanian devils.
Examples of Marsupials
Monotremes or egg-laying mammals are a group of mammals that form the order Monotremata.
Monotremes are the only mammals that lay eggs, but they also feed their babies with milk. The word ‘monotreme‘ refers to their common rear opening, the cloaca.
The platypus, the short-beaked echidna, and the three species of long-beaked echidna (Western, Eastern and Sir David Attenborough’s) are the only monotremes that still exist.
Placental mammal, (infraclass Eutheria), any member of the mammalian group characterized by the presence of a placenta, which facilitates the exchange of nutrients and wastes between the blood of the mother and that of the fetus. The placentals include all living mammals except marsupials and monotremes.Britannica.com