Zerglings form the most numerous of the zerg mutations,—their simple DNA allows two zerglings to be spawned from a single larva. The larvae get the genetic information from the 'primordial ooze' of spawning pools. Zergling replication is so efficient that they thrive even when exposed to exceedingly high mortality rates, which are greater than almost all other zerg strains.

Zerglings are the smallest zerg strain apart from the larvae. Equivalent in size to a large dog, zerglings are one meter tall and possess webbed feet. Zerglings possess sickle limbs and sharp incisor teeth, used primarily for tearing into flesh. Further mutations may be made to zergling DNA, namely the alteration of their physiology through a metabolic boost to increase their reflexes and rate of movement and the mutation of their adrenal glands, changing their glandular balance to induce a battle frenzy. These adrenal glands have been observed to be unusually large for a creature of the zergling's size. Further analysis has revealed evidence of genetic manipulation, which also increased zergling aggression, driving them into a state of frenzy. In such a state, zerglings can bring down armoured vehicles and structures with ease.

The rapid genesis modification to zergling cells allows their gestation to proceed at remarkable speed. Zerglings have tough, adaptable, leathery skin which prevents moisture loss, while still allowing fluid circulation to keep the zergling's body temperature at appropriate levels. As per their origins, zerglings have an efficient nutrient storage system. Zerglings can derive the necessary compounds for life (even water) from anything they eat. This allows zerglings to operate away from a hive cluster for extended periods of time.

Zerglings are physically adaptable creatures, able to climb near vertical surfaces and are extraordinarily fast though they are generally easy to exterminate. 

Zerglings have a complex optical structure, and their eyes have an optical aperture which allows them to see in the dark. This ability is further enhanced by a layer of tissue that reflects light, and the production of a substance which, depending on saturation levels, makes a zergling's eyes glow orange or red. This glow is an indicator that a zergling's vision has fully adapted to whatever low-light conditions it has found itself in. However, zergling eyes have also appeared as pure black in such conditions.

The zergling's spiked tail allows the creature to maintain balance while running at high speeds. The spikes that extend from the tip have no discernable purpose. Some zerglings were observed to possess venational wings, which are used to increase ground acceleration. Spikes jut from the bodies of Leviathan Brood zerglings as a byproduct of their extremely simplified DNA. These spikes are retained even when Leviathan zerglings metamorphose into banelings.