Some of the characteristics of a Mammal is they have to have a vertebra, meaning they have a backbone or spine. Also they need to be able to produce milk for their babies. Another thing that is a characteristic is they are endothermic, meaning they are warm-blooded.
Some of the characteristics I've given an example of well suit the mammals environment. How so? Well mammals live in various environments and most adapt well to most environments. Honestly a mammal needs itself, a food source, water source, and a livable environment without harsh weather conditions and they're good to go.
Subclass Eutheria, subclass metatheria , subclass prototheria are the order of the subclasses in the mammal group. Subclass metatheria are marsupials. Most females in this group have pouches. For an example a kangaroo is in this group.
A kangaroo is a mammal and in the subclass metatheria. It has a pouch over its stomach to carry its young. Macropodidae is the taxonomic family the kangaroo is in. It's genus it Macropus. The kangaroo has a long muscular tail and a smaller head used for balance.
Macropus rufus (Red Kangaroo)