Good question. People like to put things into categories to keep things organized. The living world doesn’t always fit neatly into those categories. Development of babies into adults is a good example of trying to make categories.

Think about a baby horse, cat, or dog. If you saw a picture of one, you could tell that it was a foal, kitten, or puppy, even if you didn’t know how big it was. Their heads are big compared to their bodies. Their eyes are big for their heads. They have higher foreheads and smaller noses. But these are not huge differences. Basically, mammals just get bigger as they develop. Their proportions—the size of one part compared to another—change. But they do not add whole new parts.

People wanted a different word for extreme changes, like a caterpillar into a butterfly, or a maggot into a fly. This is called complete metamorphosis. A crawling thing with a big body and many small legs becomes a flying thing with 6 long legs. The animal’s entire body is re-arranged.

Some things are in between. For example, crickets and beetles start off without wings. They look different from their parents in other ways, but those differences are not huge, like a caterpillar turning into a butterfly. This is called incomplete metamorphosis.

The only mammals that fly are bats. Do you think baby bats have wings?

If you are interested in questions like this, you may want to study biology.

The gist of it is that in regular development, the babies look like the adult. In metamorphosis, the organism post-change will look nothing like the organism pre-change. When babies look like the adults, it is called "direct development" rather than metamorphosis.

Baby mammals grow continuously and don’t have any dramatic changes. A full grown mouse for the most part looks like a larger baby mouse. However, insects and frogs they can undergo metamorphosis which is rapid and really obvious changes. An example is how a tadpole looks very different than frog. Or how a caterpillar looks very different than a butterfly.

Animals that go through metamorphosis (mainly some orders of insects) undergo a complete rearrangement of the tissues inside of the animal's body. Direct development, which is what mammals and most other animals do, have the underlying body parts grow and change shape as the animal develops, but do not go through a complete resorption and rebirth as a new body.

Metamorphosis is a developmental process whereby developing organisms pass through one or more larval stages before reaching adulthood. A classic example of this is that fertilized frog eggs become free-living tadpoles, which in turn change (metamorphose) into frogs. Most mammals (except the platypus and echidnas) retain their fertilized eggs in the uterus. The young mammal is born essentially as a miniature version of the adult, that is, it doesn't change form (which is what metamorphose means) radically as it grows up, rather it basically just gets bigger.

Mammals have a very unusual way of reproducing, in other words. Great question!