Primal hosts were a primal zerg species that evolved on Zerus. During the Second Great War, the Zerg Swarm invaded Zerus and fought the primal hosts of Brakk's pack. The essences of the primal hosts were taken by the Zerg Swarm to create the swarm host, a similar creature.
Primal hosts were deployed by Dehaka during the End War, and could be evolved from their swarm host counterparts. Sometime later, some swarm host strains began to merge with primal host strains, creating a strain with similar traits to both.