Question

Why rat, mongoose,hippopotamus,dolphin classify in one group?

Answer 1

Whether it is rat, mongoose, hippopotamus or dolphin, they are all classified in one group 'Mammalia', because they show similar characteristics. All of them have, Back bone and spinal cord, they are all warm blooded. They are haired (mostly) which provide them warmth, protection and act as sensor.

Answer 2

Answer:

Unlike most other semiaquatic animals, hippos have very little hair. The skin is 6 cm (2 in) thick, providing it great protection against conspecifics and predators. By contrast, its subcutaneous fat layer is thin.