From publisher blurb:
This sourcebook dramatically expands on the concept of the Mutant player race being a modular, free-form species built using the Fursona character builder. Fursona: The Definitive Guide to Creating Anthropomorphic Characters is required to use this sourcebook. Fursona III, which includes additional content useful to Mutant characters, is useful but not essential.
Build the type of mutants you want with this expanded suppliment built on the wildly popular Fursona framework!