When Wild animals are commonly divided into groups with similar characteristics. the two categories used for mammals what?

Eutherians, or placental mammals, are mammals that give birth to live young that have developed inside the mother's uterus. Placental mammals are the most diverse group of mammals and include humans, dogs, cats, horses, and whales. Placental mammals are characterized by their highly developed placenta, which is an organ that provides nourishment and oxygen to the developing fetus and removes waste products.

Metatherians, or marsupials, are mammals that give birth to live young that are born in a very underdeveloped state and continue to develop in a pouch on the mother's body. Marsupials include kangaroos, koalas, and opossums. Marsupials are characterized by their pouch, which is a skin fold that provides a protected environment for the developing young.

Prototherians, or monotremes, are the only mammals that lay eggs. Monotremes include platypus and echidna. Monotremes have a cloaca, which is a single opening that serves as the outlet for the digestive, urinary, and reproductive tracts. Monotremes are the least diverse group of mammals.