Typically, the only reference to Pimpl class in the header for the Owner class (Cat in this case) would be a forward declaration, as you have done here, because that can greatly reduce the dependencies.
For example, if your Pimpl class has ComplicatedClass as a member (and not just a pointer or reference to it) then you would need to have ComplicatedClass fully defined before it's use. In practice, this means including "ComplicatedClass.h" (which will also indirectly include anything ComplicatedClass depends on). This can lead to a single header fill pulling in lots and lots of stuff, which is bad for managing your dependencies (and your compile times).
When you use the pimpl idion, you only need to #include the stuff used in the public interface of your Owner type (which would be Cat here). Which makes things better for people using your library, and means you don't need to worry about people depending on some internal part of your library - either by mistake, or because they want to do something you don't allow so they #define private public before including your files.
If it's a simple class, there's usually no reason to use a Pimpl, but for times when the types are quite big, it can be a big help (especially in avoiding long build times)
