The first alien species you will likely encounter are short green men.
They tend to have low strength, health and accuracy. However, they are well known for being quick to react against incoming fire. The high-ranking aquatoids tend to use M.C.
At the beginning of the game, Gillmen are most likely to be the first ones to perform a terror attack.
Compared to the Aquatoid, they tend to be stronger but slower. They have poor reactions, allowing you time to launch the first attack if you stumble upon them.
Lobstermen are one of the most dangerous enemies in the game. Because of both their high armor and damage resistance, they are highly resistant to most forms of attack. If you encounter a group containing them, it is best to abort mission if you don't have weapons that can damage them. In addition to using standard alien weapons, they can easily run up and attack with their claws. Lobstermen are more likely to be knocked unconscious rather than killed, if hit by anything other than the most damaging weapons.
They are practically immune to the starting conventional weapons. At most, you can use the Gas Cannon's Armor piercing rounds and hope you make at least 50 direct hits. The Torpedo launcher isn't more effective, requiring at least 50 torpedos as well. If you haven't researched, your best weapon is the Magma-Pack Explosive, but you will need a lot of then.
Gauss technology fares slightly better, but not by much. The Gauss pistol requires an average of 36 hits, the gauss rifle requiring 20, and Heavy Gauss requiring 10.
These enemies only begin to fall once you obtain sonic weapons. The Sonic Pistol kills them in 6 hits, the Sonic Blasta-Rifle in 5, and the Sonic Cannon in 4. Sonic Pulsers can hit the weakest side of the Lobstermen's armor, but you may still need 5 grenades. The ultimate weapon, the Distruptor Pulse Launcher, still requires more than one hit on the lobstermen.
Despite their power, they are vulnerable to melee if you can get close to them, and you will need armor to do so. The thermal tazer will stun a lobster man in three hits. The Vibro Blade will begin to take them out in one hit, and the Thermic Lance is more frequent with the 1-hit kills. The Heavy Thermic Lance is slightly less effective but is capable of stunning them. You may also use Molecular Control on them, as they have no special defence against it.
The Hallucinoid is a terror unit paired with Aquatoids, and normally appears underwater.
It is most vulnerable against incendiary attacks
The Calcinite is a terror unit paired with Aquatoids, and normally appears in land missions. It is a fast moving unit with a powerful melee attack, but doesn't use any ranged weapons.
The Deep One is a terror unit paired with Gillmen, normally appearing in land missions. They don't carry a weapon, but have an innate ranged attack.
The Xarquid is a terror unit paired with Gillmen, normally appearing underwater.
The Tentaculat is a terror unit appearing with Lobstermen and Tasoths while underwater. It is a flying enemy, and can move quickly.
It only has a melee attack, but if it uses it, the affected Aquanaut will be turned into a zombie. Unless the zombie is killed with a incendiary round, it a new Tentaculat will pop out of the corpse.
It generally prevents X-Com aquanauts from using reaction fire, due to its high reaction attribute.
The Bio-Drone is a terror unit appearing with Lobstermen and Tasoths while on land. It is equipped with a sonic cannon.
This unit explodes when killed. However, using a melee attack may bypass the explosive result.
The triscene is a terror unit appearing with Tasoths while on Land. Despite the in-game documentation stating they appear with Lobster Men, they don't appear with them unless deployed as part of a mixed crew. It is heavily armored, providing immunity to any weak direct-fire weapons. It's weakness is the under-side armor, leaving it vulnerable to only a few thrown grenades.