Bodies are how the game conceptually organizes creatures, primarily for purposes of equipment slots. Through modding, the body of any creature is completely customizable.
The data file for body information is
Bodies.xml. It's organized into three main sections:
bodyparttypes(or types for short), the core kinds of body parts;
bodyparttypevariants(or variants for short), body part sub-types that are more or less interchangeable with other parts of the same main type;
anatomies, which are actual structures of body parts. These are assigned to objects in ObjectBlueprints.xml.