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.
The alien races are split into two categories, Armed Combatant and Terror Unit, and ordered according to their general order of appearance.
Armed Combatants
Armed Combatants are races that can operate independently and use manufactured weapons.
Gill Man
Gill Men (also spelled Gillmen) are not true aliens, but an ancient Terran species that serves them. They make rather poor soldiers, and their alien overlords seem to treat them as little more than cannon fodder.
They are slower and less accurate than Aquatoids and have no Molecular Control ability, with the sole exception of those of Commander rank. However, they wield the same powerful sonic and explosive weapons as the other alien races and as such should not be underestimated.
They have all ranks except for the Medic and Navigator.
Methods of Disposal: Pretty much anything, though weaker weapons such as Hydro-Jet Cannons, Gauss Pistols, and especially Jet Harpoons will usually not kill them in one hit. Unlike Aquatoids, they may survive when on the edges of an explosion. They are vulnerable to M.C. attacks.
You will need to have examined a dead Gillman in order to unlock the Thermic Lance. Their associates are perhaps of more interest.
See also: Xarquid | Deep One
Aquatoid
Distant relations to the Sectoids, they have much the same strengths and weaknesses.
Aquatiods make at best average soldiers, having pitifully low health. However, they do have decent Reactions and Firing Accuracy, and their Technicians, Navigators, and Commanders have Molecular Control abilities.
Aquatoids can be of any rank (they are the only race to have Medics). They usually act independently, but can also be found as part of Mixed Crew missions and the forces defending Alien Colonies and Artefact Sites.
Methods of Disposal: Anything. Explosives are particularly effective, and will often kill them even if they're on the very edge of the blast radius. Weapons with high rates of fire are another good option. The lower ranks are fairly vulnerable to M.C. attacks, the higher ones less so.
See also: Hallucinoid | Calcinite | Mixed Crew
Lobster Man
These orange instruments of doom are the Muton of TFTD. They are usually encountered from April 2040, but can occasionally appear as early as February.
They are very tough and flexible opponents as they can fight with standard weapons (they have decent Firing Accuracy) or their built-in claw attacks, which can give even the most well protected aquanaut a major pounding.
While their armor is weak, they have insanely formidable resistances that allow them to shrug off between 50% and 80% (!) of most damage types. Combined with plenty of health, this allows them to sustain unreal amounts of damage before dropping dead. Thankfully, this is balanced by Lobstermen taking more damage from stun or drill type weapons.
Lobster Men have all ranks except for Medic, and none of them have any M.C. ability at all. In addition to running their own missions, they also man the lower levels of Alien Colonies.
Methods of Disposal: Thermal Shok Bombs (or Tazers if you're feeling brave) and drills (e.g. Vibroblade, Thermic Lance) are ideal. Failing that, use lots of powerful explosives or Sonic weapons with spare clips. M.C. attacks are effective against them, though the higher ranks are somewhat resistant. Gauss and beginning-tier ranged weapons are ineffective.
A live Lobster Man Commander is required to research the Leviathan Assault Sub and T'leth, the Alien's City, both of which are required to finish the game. Earlier versions of the game required researching a Lobsterman Navigator for Magnetic Navigation.
See also: Tentaculat | Bio Drone
Tasoth
Tasoths are alien super-soldiers, with a deadly combination of high TUs and Reactions as well as good Firing Accuracy and very high Health. While they lack the damage resistances of the Lobster Men, they do not have any glaring weakness, either. They usually start appearing in July, but may sometimes be encountered earlier, in Terror Missions, Alien Colonies or Artefact Sites.
Tasoth come in two flavours: Soldier and Squad Leader. The Squad Leaders have significantly better stats and, more importantly, Molecular Control ability. In addition to running their own missions, they are also found defending Alien Colonies and Artefact Sites and comprise the majority in Mixed Crew missions.
Methods of Disposal: Sonic weapons are preferred, but strong explosives and melee weapons will also do the trick. Gauss Weapons will struggle against them, and starting-tier weapons are generally ineffective, save for powerful explosives such as the Torpedo Launcher. Tasoth Soldiers are vulnerable to M.C. attacks, Squad Leaders less so.
A live Tasoth of either rank is required to research the M.C. Disruptor.
See also: Bio Drone | Tentaculat | Mixed Crew
Terror Units
Terror Units are always deployed to support Armed Combatants, and never run missions of their own. They have more specialized roles and only use built-in or natural weapons. They do not have ranks other than Terrorist.
Deep One
Deep Ones are fire support aliens with a unique and powerful arcing weapon that they can fire over obstacles. Sadly, it cannot be recovered or used by X-COM. Otherwise, the Deep Ones combat capabilities are unimpressive, as they are slow, rather fragile, and not very accurate.
They are found in the company of Gillmen on land missions, and are therefore likely to be the first Terror Unit encountered by X-COM. Interestingly, they appear underwater only in the final mission of the game.
Methods of Disposal: Anything stronger than harpoons will kill them easily. They are resistant to M.C. attacks, though not as much as the other terrorist species.
It cannot be overstated how vitally important these creatures are to X-COM research. A dead body is required to unlock research into Aqua Plastics and Plastic Aqua Armor, and afterwards a live specimen is needed to unlock Ion Armor, advanced submarines and advanced SWS models.
See also: Gill Man
Calcinite
Melee only units with powerful claws. They can take a bit of a beating due to their 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.
Calcinites are found in the company of Aquatoids and Mixed Crews on land missions. While Calcinites may be encountered as very early in the game, their Aquatoid masters are less inclined to launch Terror Missions than Gill Men, meaning that in some playthroughs X-COM will have to wait a long time before facing one.
Methods of Disposal: Gauss or Sonic preferred, but Explosives and Gas Cannons can be useful. As they are melee-only, Particle Disturbance Grenades can be particularly effective. They are highly resistant to M.C. attacks.
A dead Calcinite is required to unlock all the melee weapons, starting with the Vibroblade.
See also: Aquatoid | Mixed Crew
Bio-Drone
These are one of the most dangerous aliens in the game. They can be thought of as miniature Cyberdiscs with high TUs, very high Health, and a sonic weapon with the best accuracy in the game. Not only can they take a considerable amount of punishment before falling, but they also explode if a ranged weapon kills or stuns them. Also, like Cyberdiscs, they can fly.
Thankfully, they can be detected by a burned trail they leave behind them as they move. In addition, while they have a melee attack, it is extremely inaccurate, and they will often waste time trying to hit any aquanaut standing adjacent to them.
Bio Drones are found on Lobstermen, Tasoth, and Mixed Crew land missions. As such, X-COM will probably first encounter one on 1 April 2040. They do not appear underwater.
Methods of Disposal: Either Sonics from as far away as possible (other guns will work, but not as well), or drills as close as possible. They are highly resistant to M.C. attacks.
See also: Tasoth | Lobster Man | Mixed Crew
Hallucinoid
The Hallucinoid is a large, floating alien that primarily attacks with a lethal "cold-based" melee attack. It does not have a ranged attack, despite the UFOpaedia entry, and attacks only with its tentacles (which deal ordinary melee damage, not Freeze damage).
Hallucinoids are found in the company of Aquatoids and Mixed Crew on underwater missions, and also defend Alien Colonies and Artefact Sites. They are not found on the surface.
Methods of Disposal: Sonics or powerful explosives are preferred. Most other weapons will work, but on higher difficulties it will take many shots thanks to Hallucinoids' high health and considerable armor. They do have a weakness to Phosphor rounds, but Phosphor is so ineffective underwater that it's usually not worth the trouble. They are highly resistant to M.C. attacks. As they are large units, one can stay out of their reach simply by moving through a single-space door or in other spaces they cannot reach.
Hallucinoids are able to alter their depth (i.e. "fly"). This can make it harder to hit them thrown explosives or melee weapons. On the other hand, it makes misses with ranged area-effect rounds much safer.
See also: Aquatoid | Mixed Crew
Tentaculat
Tentaculats are extremely dangerous aliens, very similar to the Chryssalids of the first war. These big brain creatures have the same ability to proliferate by zombifying their targets into mindless walking incubators for their young. Tentaculats are able to raise or lower their elevation in the water with impunity. In short: underwater flying Chryssalids. The one saving grace is that no civilians are ever encountered on the same map as Tentaculats.
Tentaculats have a high distribution of front armor, but their rear and sides are weaker. If encountered early with weaker weapons, try to hit them from the flanks or rear as much as you can. Liberal use of explosives generally does the trick.
They are found on Lobsterman, Tasoth, or Mixed Crew underwater missions as well as guarding Alien Colonies and Artefact Sites.
Methods of Disposal : Sonic weapons, explosives, and a healthy dose of luck. Gauss is less effective, harpoons even less so. Submersible Weapons Systems cannot be zombified, and Displacers are almost immune to their attacks. Particle Disturbance Grenades can be effective, but a Tentaculat at full health will often survive a single blast. Tentaculats are almost immune to M.C. attacks, but using M.C. to control other aliens for scouting is effective on maps with Tentaculats, as that will keep X-COM Aquanauts safe, and aliens cannot be zombified.
See also: Zombie (TFTD) | Lobster Man | Tasoth | Mixed Crew | Chryssalid
Zombie - Fallen Aquanaut
The unfortunate result of human entanglement with a Tentaculat.
Zombies are slow and armed only with a (strong) melee attack. This makes them easy to evade, run away from and kill. They leave a nasty surprise upon their death: A newly emerged Tentaculat with full TUs and a hunger for tasty X-COM flesh. Striking the killing blow on a Tentaculat with a Phosphorous attack will interrupt the hatching process, and no Zombie will emerge from its corpse.
Zombies can only be found where there are already Tentaculats which (thankfully) means that X-COM doesn't have to worry about Civilians feeding these things.
Methods of Disposal: Ranged weapons preferred over melee weapons due to the danger of being too close, though melee weapons are probably the most effective if one is willing to risk being next to a dead Zombie when the Tentaculat emerges. HE is good, Sonic, and Gauss are fine, AP is weak. It is not wise to kill them if one cannot also eliminate the resulting Tentaculat. If this is not possible, it is better to flee and let the Zombie pursue, as they are very slow.
There is no in-game UFOPedia entry for this creature, as they cannot be captured or researched - not even corpses can be recovered. The grainy image above is from X-COM tactical video footage.
See also: Tentaculat
Xarquid
One of the less common TFTD units. They resemble a giant nautilus that move backwards, with their tentacles facing the front.
They have decent stats they don't seem to move around much, preferring instead to 'camp' a particular location and use their good reflexes to tag intruders with their particle cannon, which is roughly equivalent to the Sonic Cannon, making it fairly decent at cracking X-COM armor. Xarquids are one of the few aliens with decent armor ratings but, being large units, are very vulnerable to explosives or stun damage.
Xarquids are normally found in the company of Gillmen on underwater missions, and occasionally in the company of Mixed Crew on land missions (likely a bug or design oversight).
Methods of Disposal: Explosives and drills preferred. Sonic will work, but multiple hits are required. Gauss and harpoons are ineffective. They are almost immune to M.C. attacks.
See also: Gill Man | Mixed Crew
Triscene
Another rare unit, the Triscene can be a formidable foe as it combines high TUs and Health with very strong armor as well as having a powerful, accurate sonic attack backed up with a very strong melee attack. It is roughly equivalent to the Sectopod of the First War.
While a dangerous foe, its biggest weakness is its non-existent underside armor, which means it can be be felled with almost any explosives, sometimes just a single Magna-Blast Grenade.
Triscenes are found in the company of Mixed Crews, exclusively on land missions.
Methods of Disposal: Explosives. Powerful melee weapons can work, but approaching the Triscene is risky. Sonic attacks are relatively effective on the lower difficulty levels, but even Sonic Cannons are mostly useless on Superhuman. Triscenes are almost immune to M.C. attacks.
See also: Mixed Crew