Sectoid

Sectoids are humanoid aliens that appear in the XCOM video game series. These creatures serve as the light infantry of the alien invasion force.

XCOM
Sectoids are humanoid creatures with sickly grey skin, gangly arms and legs and bulbous heads with no mouths.

These aliens are weaker than others and die quickly against experienced soldiers, but are still dangerous early in the game's campaign. They are usually found in groups of two or three, alongside other enemies like Thin Men or Floaters. They are intelligent enough to use cover and flanking tactics, unlike some aliens. As the campaign progresses, they are largely phased out in favor of more dangerous foes.

According to autopsy reports, their unusual structure suggests that Sectoids are genetically manipulated and their identical genetic info implies that they are cloned.

They also posses the ability to Mind Merge with one another, increasing their health and combat effectiveness. However, one of the large downsides to this ability is if the Sectoid that initiates the Mind Merge dies, the other linked alien soldier dies as well.

Sectoid Commander

During alien invasion there were also higher ranking Sectoids who wield greater psionic powers than their lesser counterparts. These powers are a result of such major genetic and cybernetic engineering that the physical appearance of Sectoid Commanders is easily distinguished by their enormous head that is covered by thick veins.

Extra psionic capabilities include ability to directly invade human mind, fraying it with neural damage or induce panic, or even gaining control of it's actions.

Their abilities not limited to their own species any more, their Mind Merge ability can also enhance most alien creatures.

XCOM 2
In the years since the aliens have occupied Earth, several of the alien species have undergone significant changes, including the Sectoids. They look much more imposing than before, being taller and more muscular as well as possessing a visible mouth and teeth. They also have pink-hued skin, though they still have glowing chests like they did before. These changes have been brought about by introducing human DNA to the Sectoid gene pool.

While stronger than they were during the initial invasion, Sectoids are still among the weaker alien units encountered by XCOM. They possess twice as much health as they used to and carry wrist-mounted plasma blasters into combat. The most remarkable difference, however, is the addition of psionic powers which were previously only wielded by Sectoid Commanders, but have since become standard among all Sectoids. Using their Mindspin ability, Sectoids are capable of inducing panic in human enemies, disorienting them or even outright controlling them like pawns. Their most terrifying ability, however, is their ability to reanimate dead XCOM or ADVENT soldiers as Psi-Zombies.

The Bureau: XCOM Declassified
In this spin-off third-person shooter, Sectoids serve as advanced scouts and slave labourers for the Outsider invasion force. They look much like their mainstream counterparts, the only noticeable difference being a metallic collar around their necks that the Outsiders use to control them. During the game, there are several scenes to suggest that Sectoids are looked down upon by the Outsiders; one such scene displays Outsider soldiers bullying and tormenting a Sectoid servant and laughing at its pain. Sectoids can also be seen imprisoned in Outsider detention cells.

In combat, Sectoids will scamper to and from different points of cover. Early on, they fight using laser pistols, but later they will use heavy plasma pistols that deal greater damage to the player's squad.

Trivia

 * In the original UFO: Enemy Unknown game, Sectoids had mouths.
 * The Sectoids' appearance is clearly inspired by the classic Grey aliens.