The alfgan was a variety of animal lifeform that could be found inhabiting the environs of the artificial planetoid-starship of Chatalia. The alfgans were herded by the vessel's Chatalian inhabitants for thousands of years during their journey to a new planetary home.
The 256 families of Chatalians each identified with a certain caste and an assigned function within their civilization. When the alfgan stopped breeding and their population declined, the family of alfgan herders began to decline also. When the last of the alfgan and their herders had died, no new members of the alfgan-herder family were cloned by the Father Machine of Chatalia. (TOS novel: World Without End)