What cartoon that the characters transform into animals in their body?

You're likely thinking of "The Powerpuff Girls".

Here's why:

* Transformations: The Powerpuff Girls, Blossom, Bubbles, and Buttercup, have the ability to transform into various animal forms.

* Within their body: The transformations are not external suits or costumes, but rather happen within their bodies. They gain animal abilities like flight, enhanced senses, and physical strength.

Let me know if you have any other clues about the cartoon, and I can try to help you narrow it down!