well im gonna write here which alien races i know zerg,protoss,eldar,tau,necrons,dark eldar,orcs(warhammer 40k),krogans,turians,quarians,salarians,reapers,geth,asari,trogg(quake),sectoids,snakemen(xcom),floaters,cyber disks,reapers(xcom),chryssalids,(forgot the names of the purple men in xcom and the alien mech),combine,headcrabs,bullsquids,vortigaunts,alien grunts(half life 1),gargantua,nihilanth,pitworm,shock roaches,the floating metal spider like race in crysis,greys and etc