The mutants we call "Brainmen" are almost certainly originally human, but its present form has very little humanity indeed. It has a large head with highly developed psionic centers. In other respects its anatomy starkly differs from that of humans. Its food-intake opening is located in his belly, surrounded by sharp, needle-like tooth.
Brainmen are vulnerable to attacks by conventional weapons. They can attack psionically at a distance, stunning our soldiers and then attack either by stinging cells at the tips of their fingers or by their teeth.
They are very HP-bloated in all stages of the game.
The Morelmen were mutants that are said to have appeared shortly after the alien invasion. Most of the mutants of that time were sterile and eventually died out. However, Morelmen have devised a way of creating progeny: the parasite that "animates" the body can create a "germ" that can infect human corpses and mutate them into Morelmen.
The Morelman thus consists of the human body and the parasite that is hidden in the large "hump". The parasite connects to the gastrointestinal tract of the host and also preserves its cardiovascular system.
The Morelmen attack either by squirting acid from the chest opening, or, more often, they simply use light human weapons: pistol, shotgun, grenades.
They are vulnerable to conventional weapons but at anystage later than Cultist Appearance they are very HP-bloated.
The Flatsters probably evolved as mutated animals. Unlike most other mutants, they are capable of reproduction. The usually attack in packs and they are quite obnoxious.
The autopsy showed that they have very simple internal organs, similar to Earth mollusks, even if there are important differences (Flatsters are larger, breathe air, etc.)
Flatsters are highly resistant to conventional weapons and energy weapons are of limited effect against them. Being absolutely devoid of intelligence, they are also unaffected by psi attacks. They attack by electrical discharge.
Luckily they are quite vulnerable to the Reticulan pistol and rifles.
The enemy called Muckstar appears also in the old tales from the time immediately after the alien invasion. It is not clear if the Muckstars we see today are identical to the Muckstars of yore and we very little about the Muckstar ecology.
The Muckstars look like large spiked balls, hovering above the terrain. Their autopsy reveals that the largest part of their body is filled with hydrogen that makes them float. The rest is occupied by organs capable of developing strong electric current used to electrolyze water and also to attack its prey.
Muckstars are very light and highly maneuverable, however they are also very poorly protected. They attack by electric discharge.
NOTE: unmentioned, but data codes show Muckstar ignore the dark condition outright, so are deadly at night.
These creatures, known as Scuttlebugs, appear only in Cultist territory, in the areas covered by the Biomass. Their origin is not clear, but there is some evidence that these are not real mutants, but rather beings introduced by Reticulans as a part of their plan. We speculate that their original purpose was to protect the Biomass and indeed to this day they attack ferociously anybody who enters it.
Their small, crab-like bodies aren't very resistant, but it's hard to hit them because they move so rapidly. Since they tend to attack in large numbers, the use of area-effect weapons is recommended. This is the best way to dispatch large numbers of them in a short time.
The Biomass formations we came to call Atolls are encountered almost exclusively in Cultist villages where the largest swaths of Biomass are preserved. Most researchers agree that Atolls evolved recently and were not original feature of the Biomass.
They are not creatures in true sense of the word, they are merely an organ of the larger creature—the Biomass itself. Their function is to protect and to supply the Biomass with fresh carcasses that can be metabolized.
The Atolls attack by releasing clouds of toxic gas and are extremely dangerous at close range. They are vulnerable to attacks from conventional weapons, but highly resistant to the attacks from energy weapons.