Muton

Mutons is one of the sentient races that fight against X-COM during First Alien War. Although they're sentient, the Mutons are merely foot soldiers and rely on telepathic commands of their master, the Ethereals. If a Muton was separated from the telepathic link, it will die from mental system breakdown.

Mutons are a physically strong race, with numerous cybernetic implants which enhance the cardio-vascular system and the senses. The green "skin" appears to be an organically created protective armour which is grafted onto its purple body, this makes armor-piercing weapons ineffective against a Muton. The reproductive organs were surgically removed, rely on the cloning method, like other races.

It's clear that Mutons are a warrior race and nothing else. While they are indeed skillful combatants, the fact that Mutons are controlled by the Ethereals became their weakness. First, the Mutons have relatively low intelligence and a short attention span. Second, the telepathic link makes them vulnerable to psionic attacks.

Mutons have a particular appetite for consuming raw flesh of any kind, which they need for sustenance like earth based carnivores. Since the Mutons are controlled by Ethereals, their hierarchies are made purely of low ranks soldiers, engineers and navigators.

Mutons is the only race known to be accompanied by two types of terror races, the Celatid and Silacoid.

After the Ethereals established their dominion over Earth and humanity, Mutons are genetically enhanced through Human DNA spliced into their genome, reducing their size but improving their agility and intelligence, making this species significantly more dangerous than during the initial invasion. When Humanity overthrew the Ethereals and took Earth back, the remaining aliens were left stranded on Earth. Now free of the Ethereals' control but without a means of leaving Earth, the Mutons and other species were accepted as residents of Earth alongside Humans. One Muton, Axiom, joined XCOM as a member of Chimera Squad.