From the Back of the Book:
They are impossible creatures... Their very existence violates the law of nature... Yet they do exist and they walk amongst us.
Neos, Gammas, and, strangest of all, the Mutts. They are the offspring of unlikely matings between different species of Beta. They combine aspects of their parentage, with elements of humanity.
The Hybrid Sourcebook includes new rules for creating and playing Beta cross-breeds, as well as background on Beta culture and the leaders of Beta society and alien animal Betas.