According to Abathur, the primals stole the hydralisk's essence from the Swarm, and used it to produce their own hydralisks. They were among the first of the Swarm zerg breeds to be adapted into the primal zerg genome, and the evolutionary strength of the hydralisk's essence meant that many of their traits were retained among the primals.
Some primal hydralisks develop mutations that exhibit an increasing number of lurker traits, with many resembling more lurker than hydralisk.
Primal hydralisks are very similar in appearance to normal hydralisks, though they are slightly larger and have greenish-gray carapaces. In the Heart of the Swarm single player campaign, their stats are almost identical to those of their Swarm counterparts.
Primal hydralisks in Co-op Missions have more health than their Swarm counterparts.
Heart of the Swarm
Primal hydralisks appear as enemies, and later allies on Zerus. They also appear as part of Dehaka's pack during the attack on Korhal.