Arachnoids are a sentient race evolved from arthropods. It appears that Mental has tampered with their genes, as Arachnoid legends state that a god came from the heavens, gave them a five-fingered right arm and told them to fight off any race it declares is an enemy. They have a very violent culture in which struggles for dominance frequently take place. An Arachnoid's age can be measured by the color of its exoskeleton: young Arachnoids have yellow exoskeletons and adults have red exoskeletons. This exoskeleton is somewhat resilent to small arms fire and can sustain them from a certain amount of damage, though they still can take moderate damage from rockets or grenades. The humanoid part of the creature has a hand on its right arm, equipped with a chaingun and techno-magical ammunition replenisher, and a pincer on its left arm. Arachnoids come in two variants: juvenile, and adult. Juveniles have yellow skin while adults have red skin, are larger, have more health and fire in volleys twice as long compared to the juveniles.
The Highlander's Bride is a female Highlander Aludran Reptiloid. She is more aggressive and dangerous than the male, due to her all-year-round PMS and enormous size; she has a height of 246 ft (75 m), making her taller than even the huge Lava Golem and the monster variant of The General. She is stationary whenever she appears, despite being the fastest enemy in the game. She behaves exactly like normal Highlanders, except that she has twice as much health and her punch attack does more damage.
The Beheaded Rocketeer is an executed Sirian soldier that has been resurrected from the dead with the use of Mental's cyber-zombie technology. The Rocketeer is controlled by a life control unit, which is an electronic device replacing the functionality of the removed upper part of the central nervous system. The device also contains crude memory modules and a vocal synthesizer.
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The Beheaded Bomber is an executed Sirian soldier that has been raised from the dead by Mental and implanted with a life control unit. He wears a green shirt and holds his head in one hand, and produces pocket bombs from the other.
The Beheaded Firecracker is an executed Sirian soldier that has been raised from the dead and implanted with a life control unit and vocal synthesizer.
The Beheaded Kamikaze is an executed Sirian soldier, raised from the dead by Mental and controlled by a life control unit. The head of the Kamikaze has been ruined, therefore, he is employed for suicidal attacks in the form of fragmentation grenades.
The Common Aludran Reptiloid is a medium-sized, four-handed reptilian creature originating from the Aludran system, located in the constellation of Canis Major. Mental discovered this primitive race and they agreed to join his army in exchange for magical powers, which enables them to cast magical fireballs with homing capabilities.
Cucurbito the Pumpkin is a genetically engineered humanoid who wears overalls, uses a chainsaw, and has a pumpkin for a head. He was created by Mental by combining elements his time-traveling agents found on 20th century Earth, including Smashing Pumpkins albums, a copy of the Burt Reynolds movie Deliverance and a copy of Lumberjack Digest. As Mental inserted the final, 1979th part into his creation, it sprang to life and even he faltered at the sight of it.
The Exotech Larva is a cybernetic insect that has been heavily modified and experimented on in an attempt to create an obeying bodyguard for Mental. He created the Larva by combining robotic parts with an even simpler organic creature - an insect larva. The Larva was heavily armed and put into a 'strength multiplicator' device to enhance its abilities and endurance. However, during this process, the larva tried to assassinate Mental at least 17 times. After many hours of trying to teach the Larva the difference between a caterpillar and himself, Mental gave up and decided to place his hideous creation inside the Tower of Babel to guard the Time-Lock from within.
The Demon is a large, red demon-like creature that stands up to 20 ft tall. Despite its name, the Fiendian Reptiloid Demon is actually a different species compared to the Aludran Reptiloid. The Fiendian Reptiloid Demon was originally a simple lizard-like organism from Fiendia Prime, but was mutated by Mental over several generations to create the Demon.
Gnaars are a race of omnivorous mammals from the planet Sirius. They are not very intelligent or organized, but were trained by Mental to attack opposing forces.
Highlander Aludran Reptiloids are colossal four-handed reptilian creatures with orange skin. They are more dangerous than Common Aludran Reptiloids, not only because of their large size, but also because they also have much more health, deal more damage, and will be enraged when they have been damaged below a specific amount of health, causing them to shoot 6 fireballs per "burst" instead of 3. They're also much faster than Common Aludran Reptiloids, and they can catch up with the player even if they spawn far away.
The Kleer is the reanimated skeleton of an extinct race of creatures from the planet Kleer, of Alpha Canis Majoris. They existed peacefully until Mental dropped ten billion gallons of napalm on their planet, effectively wiping out the entire population.
Kukulkan is the Mayan god of wind. Unlike other bosses encountered in the Serious Sam series, Kukulkan is not associated with Mental or his forces, but rather as the guardian of the first Time-Lock. He is also immortal due to being a god, and thus cannot be destroyed. However, after he has been defeated, he will disappear and grant access to the portal as a sign of respect to the victor.
Lava Golems are piles of lava that have been magically brought to life. While the first Lava Golem is a guardian summoned by the collected elements, the rest of the Golems appear to be made by Mental.
The Major Bio-mechanoid is a biological mechanism that was grown in Mental's bio-tanks. Its genome is programmed to give it biologically grown mechanical parts. Separately manufactured rocket launchers are attached to designed side slots and are directly connected to its nervous system. The Major Bio-mechanoid's large head resembles a brain, but is in fact a battery containment. Thus, the Major's intelligence is limited to seek-and-destroy.
Marsh Hoppers are a species of small, amphibious-like aliens about the size of a cat. They inhabit marshy wastelands on their home planet in the system of Rigil Kentaurus. A social and highly territorial species, Marsh Hoppers live in groups, and will react aggressively to anything that enters its territory. In all-devastating havoc, all individuals try to leap at the intruder and explode by over inflating themselves. Upon exploding, toxic slime contained in their intestines splashes the target. Hoppers have very low health, making them extremely easy to kill. However, Hoppers are always released in large hordes. This makes them very dangerous, as they can quickly jump on the player in succession and inflict a lot of damage in a very short amount of time.
The Minor Bio-mechanoid is a biological mechanism that was grown in Mental's bio-tanks. Its genome is programmed to give it artificially grown mechanical parts. Separately manufactured pulse lasers are attached to biologically designed side slots and are directly connected to its central nervous system. Like its larger counterpart, the Major Bio-mechanoid, the Minor has a large head that looks like a brain. Despite its appearance, though, it actually serves as a battery containment.
Mordekai is a large, floating hooded sorcerer with no legs, wielding a wooden staff with a red crystal ball on top of it. He served Mental for over 3000 years, and was such a good servant that Mental resurrected him from the dead to serve as one his subordinates in his army once again. However, as Mordekai had been dead for some time, Mental was quite late in the resurrection progress, leaving him slightly brain damaged.
The Reeban Electro-Fish is an aquatic piranha-like fish from the planet Reeb. On their home planet, they are aggressive predators that prey on smaller underwater species. They have an effective camouflage which helps to conceal themselves whilst underwater. This way, they can attack and disorient their target with electrical attacks.
The Scythian Witch-Harpy is a female clone of a long extinct Scythian race. They have been trained by Mental to attack opposing forces in close combat and casting spells.
The Sirian Werebull is a strange mixture of Sirian and cattle DNA, and looks and behaves very much like an ordinary bull, but has hands on its front legs and hooves on its hind legs.
The Zorg Commander is a bipedal, skinless lizard-like creature. Unlike Zorg Mercenaries, Commanders have some skin left, which is the reason why they were promoted to commanders. They're equipped with a shotgun-like energy weapon as opposed to the weak two-shot burst guns used by the Zorg grunts, and have slightly more health.
The Zorg Mercenary is a bipedal, skinless lizard-like creature. Its people, the Zorgs, managed to create a large interstellar empire. After conquering more planets than they knew what to do with, they decided to take a very long break on a tropical planet in their system. They sunbathed all day from 10am to 4pm. After the years, these long sessions caused the Zorgs' skin to fall off. After their disastrous vacation, Mental hired huge numbers of them and imported them from an unknown part of the universe to act as disposable grunts, in an effort to stop Serious Sam.
Zumb'uls are a species of bipedal humanoid aliens from the planet Ras-ad-Nyk. They were trained by Mental in his 'East Clintwood' facilities, which were named after a famous 20th century gun-wielding actor. According to rumor, every Zumb'ul being trained passed because Mental threatened that they would spend the rest of their lives taking pictures of bridges if they failed. It turns out they all graduated.