A pet-only family has no money, but can be combined with another family just like human Sims. With a bizarre (but good) design decision, EA has allowed us to create a family comprised entirely of pets. The easiest way to get a feel for creating a pet is to make one in Create a Family screen. The layering techniques of pets are complex enough to warrant their own little subsection, so first we'll touch on everything else.
This section will take you step-by-step in creating a brand new breed for your Sims. Personally, 99% of the fun I've derived from the expansion pack has been creating the looks of new breeds of animals.