Unggoy share a number of features with different animal species on Earth. Like primates, they walk both bipedally and with the aid of their over sized forearms albeit in a manner similar to a chimpanzee. However, their bodies are armored with a hard exoskeleton as with crabs and insects. Their mouths have a set of small, pointed teeth, much like reptiles. However, the UNSC has classified them taxonomically as being most similar to Earth arthropods.
Unggoy are on average about three to four feet tall and are relatively weak compared to other Covenant species. Although they can easily walk upright on two legs, they may often use their arms to move in a quadrupedal fashion. While carrying armaments, they are forced to walk upright so that they may support their weapons with their hands, but while trying to flee, or while patrolling without a weapon drawn, they use their over sized arms as forelegs to add speed or stability to their gait. Their eyesight and hearing is average, and seem to have a very well developed sense of smell, sometimes sniffing the air to detect traces of foes or hidden enemies.[10] This is facilitated through the olfactory membranes installed in their masks.
Unggoy also have thick, bio-luminescent,[11] light-blue blood,[12] possibly a result of methane-based proteins in the blood to facilitate methane-based respiration. Like Earth's Horseshoe Crabs, it is possible that their blood is blue due to high copper content.
Unggoy have over-sized forearms, while their upper arms and thighs are seemingly small. This has allowed them to be quite excellent climbers, as discovered in Halo: Contact Harvest. Spiny protrusions on their elbows help them scale rocky surfaces and may also be used to add force to their melee attacks. They also have an amazingly fast finger reflex speed, as they can fire a plasma pistol of the same rate a Sangheili can with a plasma rifle.
The standard methane re-breather apparatus.
The Unggoy have flat, three-toed feet, with another toe on the back of their feet. While they are small, stocky, and lacking in agility, they are quite sturdy; most may be weak, as they seem to be unable to carry a weapon heavier than a plasma pistol, although certain Unggoy of higher ranks appear to be surprisingly strong, able to carry an undeployed plasma cannon or fire a fuel rod gun with one hand without toppling from the recoil. However, physical strength may not actually go along with rank; the types of weapons available to them may be a part of a "trust" issue with the other Covenant. As rank-and-file Unggoy are clumsy and tactically inept, more powerful weapons are likely only issued to those who are more disciplined and experienced.
Although they require an atmosphere to breathe, Unggoy are capable of surviving in the vacuum of space without sealed, full-body gear. Unggoy carapaces apparently can withstand the chilling cold and airless environment of EVA conditions, but they must employ sealed face masks in order to breathe, as well as presumably to protect their facial tissues.
Unggoy breed prodigiously, with large egg clutches and short childhood periods.[4] This combined with the large number of young per clutch and rapid sensory-motor integration development was one of the primary reasons why the Covenant cleverly used the breeding habits of the Unggoy to their advantage in battle, deploying the Unggoy as cannon fodder (this would be play a key-role in their war against humanity). Overpopulation is a serious problem and in non-war times, strict laws are placed on Unggoy breeding.
Unggoy do value their ties to their offspring, but are usually separated from their families at an early age by the requirements of military service to the Covenant. They resent this greatly, but because of their low social status in the Covenant, they are unable to muster any response.