It does not seem particularly surprising to us that we might find gill pouches, tails, or other such structures in humans. But we would be very surprised if we found hair, nipples or middle ear bones in fish, amphibians or reptiles. Why?

In the course of evolution mammals are the animals which are evolved recently as compared to other groups such as fish, amphibians, or reptiles.

In addition, fur or hairs on body, mammary glands, middle ear bone, warm-blood, et cetera are the characteristics of mammals.

These characteristics were evolved during the course of evolution; they were not present in ancestral organisms.

However, tail, gill pouches, et cetera are characteristics of our ancestral groups. Thus, characters can be sometimes observed in mammals.

But mammals characteristics can not be observed in organisms which were evolved before mammals such as fishes, amphibians, reptiles, et cetera.