Aliens

The Aliens (also known as Uglies, X-rays, Greys and many other terms) is the human term made to used to collectively designate the extraterrestrial enemy force that invades Earth at different points in time

Sectoid

 * Commander

Thin Man

 * Chimera

Outsider

 * Eidolon

Floater

 * Heavy Floater

Chryssalid

 * Zombie
 * Hatchling

Reaper

 * Bull

Muton

 * Elite
 * Berserker

Ethereal

 * The Great One