From Creatures Wiki
The Species is the third and innermost level of classification used for agents in the Creatures series, and defines each specific agent type, for instance a crobster versus a goldfish. This is in contrast to the Genus, which defines the agent's category (ie. critter, toy, food), and the Family, which defines broadly what kind of agent it is, such as a part of the user interface, a compound object, or a Creature.
The same term is also used to denote a Creature's type: Norns, Ettins, Grendels, and Shee/Geats are referred to as species. This usage is not related to the classification system as described above: the different "species" of Creature are classified as separate genera.