Primal zerglings encompass a large number of various primal zerg. Some are more suited to life on Zerus, four legged creatures with rigid spines, while others have evolved to resemble the zerglings of the Zerg Swarm, with similar combat capabilities.
Heart of the Swarm
Primal zerglings appear in the Heart of the Swarm campaign. Compared with their Swarm counterparts (without the Hardened Carapace mutation), primal zerglings have greater health and are more reptilian in appearance. Also, unlike their Swarm counterparts, primal zerglings cannot morph into banelings.
The primal zerglings first appear during the first engagement on Zerus, where several primal zerglings are guarding a primal zerg essence pool near Kerrigan'shive cluster on the orders of the primal pack leader Brakk.
Later, Brakk starts sending out raiding packs to kill the quillgor packs and destroy their meat so that Kerrigan cannot feed the meat to the Ancient One, Zurvan, and, therefore, cannot awaken him.
The primal zerglings are later aligned to one of the two packs, the remnants of Brakk's pack or to Yagdra's pack, during the second engagement on Zerus.
The primal zerglings are a hostile force in the third and final engagement on Zerus, aligned to Yagdra, Kraith, Slivan, or Zurvan's pack.