Overviews of Aliens (TFTD)
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 creatures can be in a fight. For more details, look at aliens' specific pages or the stats page.
Low Threat
Gill Man - Armed Combatant
These guys are the worst. Not only are they mediocre soldiers but they're pathetic traitors to boot. They have marginally better stats. than the Aquatoids but have no M.C. ability whatsoever with the sole exception to the Commander rank, who isn't especially good at it anyway. Gillmen have no redeeming features whatsoever from a military standpoint, and we can only assume the aliens use them as cannon fodder by testament of their frequent appearance on the battlefield. They have all ranks except for the Medic.
Methods of Disposal: Pretty much anything.
You will need to have examined a dead Gillman in order to unlock the Thermic Lance.
See also: Gillman | Medium Threat: Xarquid - Terror Unit | Medium Threat: Deep One - Terror Unit
Calcinite - Terror Unit
Melee only units with Vibro Blade claws. These guys can take a bit of a beating due to thier high level of health and are pretty fast, but they don't have any special resistance to any weapons so sustained fire of any kind will drop them. Eventually. Calcinites have no ranks and are found in the company of Aquatoids on land missions.
Methods of Disposal: Gauss or Sonic prefered, but Explosives and Gas Cannons can be useful.
A dead Calcinite is required to unlock all the melee weapons, starting with the Vibro Blade.
See also: Calcinite | Medium Threat: Aquatoid - Armed Combatant
Hallucinoid - Terror Unit
Probably the most pointless of all the aliens, this giant jellyfish despite having lots of TUs and health as well as two weapons at its disposal seems to do nothing but float around occasionally bumping into an aquanaut and attacking with its tentacles which do precicely nothing, while completely ignoring its built-in Thermal Shok Launcher. Its large size also means it is incredibly weak against explosives. Hallucionoids have no ranks and are found in the company of Aquatoids on underwater missions.
Methods of Disposal: Anything except armour piercing, but especially explosives and phosphor.
See also: Hallucinoid | Medium Threat: Aquatoid - Armed Combatant
Zombie - Fallen Aquanaut
The unfortunate result of an entanglement with a Tentaculat. Zombies are slow and armed only with Vibro Blade claws making them easy to evade and kill, they are however completely immune to stun weapons and leave a nasty surprise for you upon thier death: A bouncing baby Tentaculat with full TUs and a hunger for tasty X-COM flesh. Zombies have no ranks and can only be found where there are already Tentaculats which (thankfully) means at least you don't have to worry about Civilians feeding these things.
Methods of disposal: Pretty much anything so long as you can take care of the resulting 'surprise' as well.
Medium Threat
Aquatoid - Armed Combatant
Relations to the Sectoids, these guys have much the same strengths and weaknesses. They make at best barely average soldiers having pitifully low stats, so why are they 'medium threat' you ask? All of thier ranks except for the Soldier have Molecular Control abilities, and the higher ranks can be pretty good at it. Aquatoids can be of any rank (they are the only race to have Medics).
Methods of Disposal: Anything, Staring, Harsh Language - they really are that weak.
Deep One - Terror Unit
Probably the biggest mistake the aliens made was creating the Deep One as they are the basis from which X-COM goes about unlocking the most important secrets: Armour and Advanced craft. Altough they're pretty resistant to the basic 'bought' weapons, they're simply fodder for anything else. They are however armed with what appears to be an electrical mortar of some kind which is unaffected by armour and does not require a direct line-of-sight to its target, thankfully it is woefully innacurate. Deep Ones have no ranks and are found in the company of Gillmen in land missions.
Methods of Disposal: Pretty much anything really.
It cannot be overstated how vitally important these creatures are to your research, you will require dead examples to unlock research into Aqua Plastics and Plastic Aqua Armour, and afterwards live specimens to unlock Ion Armour, advanced Submarines and advanced SWS models.
Xarquid - Terror Unit
Probably the rarest of all the TFTD units. Although they have fairly decent stats they don't seem to move around much, preferring instead to 'camp' a particular location and use thier good reflexes to tag intruders with thier particle cannon, which seems to be roughly equivalent to the SWS Gauss Cannon making it fairly decent at cracking X-COM armour. However being a 2x2 unit the Xarquid is very vulnerable to damage from underneath. Xarquid are found in the company of Gillmen on underwater missions.
Methods of Disposal: Explosives, AP weapons (especially blades).
Tricene - Terror Unit
Another pretty rare unit, the Tricene is on first apperances a formidable foe with its twin Sonic Cannons and Vibro Blade bite along with lots of TUs and health. But it is let down by it's non-existant underside armour which means it can be felled by a single lowly Magna Blast Grenade. Tricene have no ranks and are found in the company of Lobster Menn on land missions.
Methods of Disposal: Explosives.
High Threat
Lobster Man - Armed Combatant
These orange instruments of doom are the Muton of TFTD. Big and tough, but not really the sharpest knife in the alien drawer. Despite this they are very flexible opponents as they can fight with standard weapons or thier built-in Thermic Lance attack. However, they have a formidable resistance to almost every weapon you can get your hands on and plenty of health meaning they can take a sometimes unreal amount of punishment before dropping dead. Lobster Men have all ranks except for Medic, and none of them have any M.C. ability at all.
Methods of Disposal: Stun Bombs (or Tazers if you're feeling brave), Bladed Weapons, Auto-Fire Gauss and a backpack full of clips...
A live Lobster Man Commander is required to research the Leviathan Assault Sub.
Tasoth - Armed Combatant
Fast and agile, these guys can be quite a nuisance as they have plenty of TUs to run around and lots of health to soak up damage, although they don't really have any special resistance to any weaponry. Tasoth come in two flavours: Soldier and Squad Leader. Both ranks are more or less the same except for the leaders having slighty better stats. and more importantly Molecular Control ability.
Methods of Disposal: Sonic and Gauss work fine, you'll just need more of it.
A live Tasoth of either rank is required to research the M.C. Disruptor and M.C. Generator.
Bio Drone - Terror Unit
Miniature Hovertanks with good armour, powerful and accurate weapons and absurdly high reactions. These things are a menace that can not only cut your men down in thier own turn, but take a fair amount of punishment before falling, and when they do fall they explode. There are however two quirks of this beast to take advantage of. Firstly they don't self-destruct if they are killed by bladed weapons (this includes the Thermal Tazer. And second, they have a secondary melee weapon which they prefer to use in close quarters, the thing is it doesn't seem to do any damage. Bio Drones are usually found in the company of Lobster Men for land missions.
Methods of Disposal: Gauss, Sonic and Explosives from as far away as possible, or Blades as close as possible.
Tentaculat - Terror Unit
Tentaculats are very similar to a Chryssalid. These big brain creatures have the same ability to proliferate by zombifying their targets into mindless walking incubators for their young. They are only found in water and are generally found guarding Dreadnaughts, Terror Ships or alien structures. Tentaculats are able to raise or lower their elevation in the water with impunity. In short: Underwater flying Chryssalids. Tentaculats have no ranks and are usually found in the company of Lobster Men for underwater missions.
Methods of Disposal : Phosphorous rounds, explosives and a healthy dose of luck.
See Also Chryssalid | Zombie | Tentaculat