Breed

From StarCraft 2 Wiki

Zerg breeds are different types of Zerg creatures. The term "breed" should not be mixed up with Broods.

Every Zerg gene-family is considered to be of the same breed, such as Larvae, Zerglings, Mutalisks or Overlords. There can be some differentiation between the creatures of a single breed, but any differentiation is small. While Hydralisks and Lurkers share a great deal of genes, the fact that Lurkers mutate from Hydralisks does not make them the same. They are in the same evolutionary tree, but not of the same breed.

There are about 23 known Zerg breeds, and probably countless more hidden deep in their territory (not counting any infested creatures). Not all breeds, regardless of task, are fit for life on any planet or flexible enough to be in a combat zone, so it is assumed that the Zerg have evolved other breeds that are more highly specialized that is generally not seen on any given battlefield.