Overviews of Aliens
All aliens are not, of course, equal. While a novice may be overwhelmed at any manifestation of Alien presence, he will soon learn to cope, and discern the abilities of his enemy. For those who aren't yet well-versed in such things, here's a rundown on how mean each of these beasties can be in a fight.
Low Threat
Low Threat: Sectoids
The Sectoids are possibly the first alien you'll see. They resemble the "Greys" spoken of by New Age types and the X-Files television show.
Sectoids are very weak in combat, with generally poor accuracy and reaction time. They have mediocre mobility, no armor worth mentioning, and will easily fall under standard rifle fire, grenades, or even a glancing shot from heavier weapons. Without their more devious Sectoid Leaders, can be overcome with minimal risk.
However, on any mission that doesn't involve a small-sized UFO (Small Scout, Medium Scout, or Large Scout), the Sectoids will have a Sectoid Leader in their ranks. This Leader is capable of Psionic attacks that can panic your troops or even bring them under alien control, to be used against you. In the early game when Sectoid sightings are common, this can be devastating-- XCom has not yet researched Psionics, and so is wide open to psionic warfare.
Furthermore, on any Mission involving terror units, the Sectoids will be accompanied by the formidable Cyberdisc, which adds another degree of complexity to the situation.
See also: Sectoid | Medium Threat: Cyberdisc
Low Threat: Floaters
The Floaters are the other variety of aliens you'll see early in the game, possibly before Sectoids. They are purple humanoid shapes clad in capes, floating perpetually above the ground. They are more formidable in combat than the Sectoid. They have better firing accuracy and reactions, along with a modicum of armor. They have somewhat better mobility than the Sectoid, and they are capable of floating in mid-air. They use this ability only rarely (almost never on lower difficulty settings), but it does mean that, at game start, you will see them more frequently on rooftops and other elevated positions. They rarely make good use of cover while floating above ground level. Floaters may be more likely than Sectoids to throw grenades, but hard data here is lacking.
While Floaters, by themselves, are more fearsome than Sectoids, they lack the Sectoids' two advantages: Floaters have no Psionic capability, and their terror unit, the Reaper, is mediocre at best.
See also: Floater | Low Threat: Reaper
Low Threat: Reaper - Terror Unit
Reapers are the dedicated Terror Units support of the Floater. It is a large, furry mammalian biped of size comperable to an X-Com HWP. It is armed only with its bite, which obviously can be used only when the alien is directly adjacent to its foe. It is arguably the least threatening alien in the game. Since they are so large, reapers ey are generally easy to spot and easy to hit. While they can absorb more damage than a floater or a sectoid, they are resistant only to armor-piercing fire and make a large target. They are particularly vulnerable to incediary fire, but almost any heavy weapon fire will dispatch them quickly. Their only real asset is their above-average mobility.
See also: Reaper, Low Threat: Floater
Low Threat: Silacoid - Terror Unit
The Silacoid is one of the two varieties of Terror Unit deployed with Mutons. It resembles a large pink rock that slowly rumbles along the ground, often leaving a trail of burnt terrain in its path. Their only combat ability is to make a moderately damaging melee attack, presumably involving the burning heat of its body. Its mobility is lacklustre, and the burnt trail it leaves makes it easy to track. They are somewhat well armored and can absorb a moderate amount of fire. Considering that they are usually only encountered in the mid- to late-game, the sight of this underwhelming combat unit comes as a pleasant surprise to Commanders accustomed to more fearsome prey.
See also: Silacoid, High Threat: Muton
Medium Threat
Medium Threat: Snakemen
Snakemen usually won't show up for at least two or three months into the game. As their name suggests, they are reptilian, resembling snakes with arms, slithering along on their lower bodies. They have noticeably better armor than floaters or sectoids, and are capable of absorbing signifigantly more fire than either. Their combat stats are at least comperable to Floaters, possibly slightly better. They seem a litle more willing to use grenades than Sectoids or Floaters, too. Their key flaw, however, is low mobility. They are not able to move very quickly, slower even than Sectoids.
However, larger Snakeman missions involving Terror Units can be truly fearsome, with the their terrifying Chrssalid.
See Also Snakeman | High Threat: Chryssalid
More to come ----Papa Legba 20:25, 11 December 2005 (PST)