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“Not how, why? What made them do it? You know what I think? I think they get sick of being guinea pigs you know?”
fragment of one of randomly generated mercenary conversations, from the level "Rebellion"
Trigens are one of two main enemy types in Far Cry alongside Mercenaries.
Trigens are beings that have been genetically altered by Krieger and his scientists using a mutagen designed to enhance the strength, speed, and endurance of humans. The experiments yielded good results in terms of strength and speed enhancement, but the mutation had a horrific side effect; all Trigens became horribly deformed beasts, and their minds got torn apart by an overwhelming primal urge to kill and feed.
Before Far Cry 1
Before creating the Trigens, Dr. Krieger was working in the United States Department of Defense, where he allegedly worked on what is only referred to as "tests on humans". How it relates to the Trigen issue is not entirely clear, but this event was the reason why he was expelled from Department, and became a private businessman, establishing Krieger Corp. 
Later in a public interview Krieger admitted that he had bought an island archipelago away from international trade routes, because it would be an ideal place for his research, on which - as he claimed - the future of humanity is depending on. 
Far Cry 1
“It's all part of Krieger's plan, Jack. He's cooked up a serum which enhances every organ and muscle in the recipient's body. So far he's only been using it on primates - Atleast as far as I know.”
Doyle in "Treehouse"
A little later, Krieger and his scientists created the serum; the first test subjects they used it on were primates, and soon enough, they began to exhibit superhuman strenght,  only to later on mutate. As it turned out, Krieger's serum amazingly increased physical abilities, but at the cost of mutation of the user and a significant increase in their level of aggression, which made all Trigens very dangerous to work with.
They were at first kept in containment areas and cages in various installations throughout the islands, being studied on by Krieger's scientists, both inside laboratories and special areas reminiscent of prisons, with tall fences blocking them from escaping, and tree-mounted walkways to keep Mercenaries safe, while all failed Trigen corpses were fed to sharks.
Even before escaping, Trigens were making many Mercenaries feel uneasy and afraid; many of the Mercenaries even had a feeling they were going to escape, and some speculated that scientists also allegedly make meat out of Trigen corpses and were feeding them with it in cafeterias. 
Soon enough, the event that sparked first escape of Trigens happened - Jack's destruction of the communication center. The scientists wanted the specimens to be kept alive, even if they were loose and killing personnel, much to the horror of the mercenary force.
Afterwards it only got worse, as more and more Trigens began to escape, especially after Jack released many of them, previously kept in cages, leading to them having an all-out war with the Mercenaries.
At this point the mercs began to kill Trigens to stay alive and defend themselves - even using specialized anti-Trigen weaponry such as the Nerve Gas - but their efforts were in vain as the mutants soon overwhelmed the islands and celebrated in victory.
Overall, the game implies that the whole process had three stages - Stage One, during which the first, non-human Trigens were created, Stage Two during which the development of human Trigens begun, and finally Stage Three. In Factory, Doyle mentions that during Stage Three Krieger got in such hurry, that he decided to take a lot of different live specimens, even going so far as creating Trigens out of zygotes, to create "vat-grown soldiers ready to fight as soon as they are born".
After Far Cry 1
After Far Cry 1's action, all Trigens presumably died out and became extinct due to being unable to reproduce, as mutants and artificially created organisms tend to.
There are overall five types of Trigens encountered in the game, of which two come from chimpanzees and gorillas, and three that are derived from humans.
Trigens seem to have a simple hierarchy based on size. Bigger Trigens will give orders to smaller Trigens when searching for Jack. They can be divided into two categories - Small, and Big (Human-Like).
Small Trigens, dubbed Monkeys by most of the characters come in two variants - the first one is a mutated chimpanzee, and the second one is a gorilla. Both are small and at first glance difference between them is purely cosmetic, althrough upon closer inspection it can be noticed that gorillas have slightly more health and deal a bit more damage than chimps, at cost of also slightly shorter jump length. Both types are small, but can shred through even a fully healed and armored Jack with just a few slashes, and are incredibly fast. They can leap upon a foe from meters away, and surprise enemies by moving through vents due to their small size. Chimps seem to be one of the most horrendously deformed Trigens, barely even resembling primates anymore, with reptilian-like muzzles, four yellow eyes, three-fingered arms and two-headed toes, with the only hair left being on their backs.
Gorillas are also barely resembling apes - they became totally hairless, their heads and jaws fused with their chests, and their deformed faces have bulging, bloodshot eyes. Their legs have two toes, and arms turned into giant, praying mantis-like claws. They are also called Aberrations.
This Trigen variant is also the first mutant seen in which it attacks and kills a lone mercenary who was running for his life. They appear in Research (cutscene only, gorilla), Treehouse, Bunker, Steam (chimps only), Control, Rebellion, Archive, Cooler, Catacombs and Dam.
Fast Trigens resemble humans, ableit with abnormally large and muscular legs. They normally carry fully automatic M4 rifles, and have the ability to leap incredible distances. They move quite fast, and will swipe/kick at enemies that get too close, though this type of mutant as the weakest melee damage.
If a player happens to shoot a Fast Trigen when it is about to leap to another location, the Locust will struggle to jump, this is an incredible weakness of them, and gives the player a higher chance of killing one.
They seem to be the most intelligent Trigens out of all of them, in cutscenes they are even shown releasing other Trigens and rallying them to wreak havoc all over the islands.
Stealth Trigens are rarely encountered Trigens are fitted with experimental cloaking devices that allow them to become completely invisible, and they put this to good use. They will often remain cloaked until they are killed, but using the biothermal CryVision goggles makes them visible. They are armed with silenced MP5s, and will claw enemies that get close. They appear in Regulator (cutscene only), Archive and Catacombs.
According to what Doyle says in Archive, they are failed test subjects. They are technically the first human mutants player sees, as they can be seen in the first seconds of the Regulator cutscene, even before the first Fast Trigen shows up. They are not seen after Catacombs, making them the least appearing living trigen.
One of the taunts of a Fat Boy
Fat Boys also called Mutant Big in game files - these big, hugely muscular Trigens are extremely tough. Most times when they are encountered their right arms are mounted with missile launchers, but rarely, they are encountered with just their bare hands to fight with. When Jack gets too close, they will strike him with their claws, taking a severe amount of health away from the player.
Fat Boys have a glowing implant in their chests that devastates them when it is shot. These are the only Trigens that can speak, and they will growl things such as "You're goin' down!" and "Its all over for you!" when in combat.
Ironically, they are the least intelligent of all the Trigens, as best seen in the occasional instances where they may get stuck in a doorway and instead of taking another way around, will attempt to get free and kill Jack by repeatedly firing their launcher, killing themselves with a blast to the face from their own missiles in the process, since their shoulders are too wide to allow their arms (and thus the launcher) to fit through a doorway. If presented with a situation in which they cannot reach Jack, they will simply shoot in his general direction until something happens. They never flank enemies and always go for them head on. Should one of their fellow Fat Boys die, they will sometimes say "Someone’s killed him!" even if they are seen drowning. However, despite their shortcomings in common sense, they do tend to patrol, inspect areas that have unusual sounds, and will alert the other Trigens. They are also very deadly in confined areas, especially if Jack encounters a group of them.
They are characterized by their loud, lumbering footsteps.
They were originally supposed to posess different weapons, such as homing rockets miniguns and shotguns.
The last type of Trigen is Krieger himself, encountered in Volcano. He can make high and long jumps, can become invisible, throws grenades and swipes at you if you are too close to him. Like Fat Boys he has a very high amount of health and can withstand several dozen bullets before falling, tho one can use his buggy hitbox to fire at his crotch area to deal massive damage, killing him almost instantly.
Krieger is very bugged, going so far as literally possessing no grenade throwing animation, just "spitting" them out of his chest.
Omega and Slug/Worm Trigens
“Congratulations, you have found one of the Omega-strain Trigens, Jack. That thing is a masterpiece of genetic engineering, it's the culmination of Krieger's research. The ultimate supersoldier! From the current data I processed...looks like he's turning him into some sort of mobile artillery.”
Doyle's unused sound file
Inside the game files are leftover files of Three unused mutants - MutantOmega and MutantWorm, one resembling a Fat Boy but much bigger (taller than Screwed Trigen) with two additional rocket launchers, and one looking like a giant snail and worm combined. They are both untextured and there are no existing models other than unused holograms.
Early sound files that indicate that the player would see an "Omega-strain Trigen" in Factory can be found in the "compound" folder, from compound specific F3 to compound specific F7. The Trigen is described as being in some sort of sleep.
Interestingly, MutantWorm has some form of leftover AI in the game - an entity called "Worm" can be placed in the Sandbox editor, but lacks any model. Once given a model, it will follow the player rapidly (to the point of glitching model's legs) but will not attack. Omega can be placed too, but once given a model, it has no AI reaction, bit like Mutant Screwed.
Also, a mod named "BRP: Beta Restoration Project", a mod to recreate the Far Cry Beta in the website Moddb, show the animations of Slug Trigen. and shows how the Omega Trigen looks like.
The Omega Trigens in combat they fire rockets and shotguns in the arms, also they shoot mini-rockets in the shoulders.
TCover, Rear and Scout Trigens
Game's coding also has references to three other unused mutants - MutantCover, MutantRear and MutantScout.
MutantCover and MutantRear posess textured models of their corpes in-game, with MutantCover greatly resembling the Fat Boy, albeit with slightly different design and a homing rocket launcher (according to in-game unused weapon coding called "CoverRL"), and MutantRear looking like a human with a prolonged neck.
It is possible that the MutantCover and the MutantScout were later turned into the Fat Boy and the Fast Trigen respectively, while MutantRear became the Stealth Trigen - this is hinted at by the fact that MutantRear is programmed to act like a Stealth trigen, while MutantScout is programmed to be a Fast Trigen, but both have unfinished programming.
The Rear Trigen art are found in "BRP: Beta Restoration Project" mod.
One of the most known cut mutants, is the one labeled as "Screwed" or "Mutant Berzerker". Along with Omega, it is the biggest mutant, with a big blade on the right arm. It also has a big mouth on its face. It is heavily implied that it's the missing link between Stage One (primate) Trigens, and Stage Two (human) Trigens.
It is not cut completely, as it's seen in Bunker on a table and in the cutscene preceding Dam. In Bunker, a Lab Worker was performing surgery on a Mutant Screwed but the operation was halted by Jack Carver.
Mutant Screwed can be placed in the Sandbox editor, but has no animations. It's behavior consists of moving after player and glitching up while in melee range, dealing massive damage much like the small Trigens.
Other cut Trigens
The game was also going to feature two more primate-based Trigens, including a baboon and another, "flat-faced" Chimp model of which can be seen as the hologram in Cooler and Volcano.
- Recently, a ModDb user discovered the site http://www.maristov.com/, which contains actual renders of Mutant Omega and Mutant Rear, and how they were supposed to look in-game.
- Fat Boys seem to switch between two voices - one being lower in pitch and composing mostly of yells, damage and death sounds, and another one, much more growling, harsh and human-like, able to even form sentences. The second voice, still called Mutant Cover in game files has a full, almost unused voiceset similar to that of the Mercenaries.
- All human-like Trigens, excluding Krieger, seem to lack lips.
- Despite appearing first time in Research Trigens are actually foreshadowed as far, as in Training in the Mercenary camp, appearing on some photos. Some mercenaries also vaguely talk about them, even speaking about a "huge rat" that seemed to be a "mutant".
- Despite Trigens being fleshy-red in colour, according to Doyle in Dam, one of the first signs of turning into a Trigen is your skin turning green.
- The Trigens bear some resemblance to several enemies from other popular videogames of that time. The simian-type Trigens resemble the demons from Doom 3, the chimpanzee Trigens resemble the Fiends from the original Quake, the human-type Trigens resemble the Strogg race from the Quake series and the Fat Boys resemble a cross between the Cyberdemon from the original Doom games and the Tank from Quake 2.
- Due to that Far Cry 1’s sandbox editor gives much more options for the designer, the trigens, respectively Fat Boys, Fast and Stealth are able to be equipped with other weapons, than their regular weapons.
- Fat Boys can be equipped with CoverRL and Shotguns even machine gun (called WPN MG in the Database console). CoverRL will make the rocket follow the player until it hits the ground or another obstacle until it explodes.
- There are voice packs for the cut trigens, they can be equipped on any Trigen in the Map Editor, simply by changing the voice pack.
- Following packs exists are:
- AB1 Which are the regular monkeys, but contains unused sounds which can be added in.
- AB2 which one of the sounds are used for the Research introduction of the Monkey in the cave.
- COVER which can be considered the mercy taunts, yells, and commands but in a deep mutant like voice, some of Fat Boys sounds are used from that pack, like the yells, pain and stomps.
- FAST which can talk like Fat Boys, but in a bit higher pitch and was made into the current Mutant Fast sounds, however they lack speaking ability.
- There’s also in the folder were the latters can be found, sounds of a pig that sounds bit mutated, possibly considered to be used on the various boars that can be found in the game.
- Following packs exists are:
- Some Fat Boys seems to be failed experiments, should a player/designer place Mutant Big (Fat Boys in-game Editor name) Archive 1, Mutant Big Archive 3 and Mutant Big Volcano, other Trigens will start attacking the 3 mentioned.
- The Mod "BRP: Beta Restoration Project " is a mod to recreate the Far Cry's version: possibly Alpha or Beta, in the mod site Moddb, show the the cut trigens are look like.
- The Cover Trigens can use a huge M249 Machine Gun and Rocket Launchers in their arms.
- Omega Trigens's Rocket Launcher on shoulders is teleguide missles that can damage Jack's Health and Armor.
- During in Far Cry beta, is revealed that Fast Trigens can use Akimbo P90s in same time.
- There is a theory involved the Human-Trigens: this theory is from FCLAB Discord, that the human trigens are actually islanders, who become Trigens. Confirmed on BRP: Beta Restoration Project, the islanders are humans who possibly be killed by the Krieger's mercenaries and/or one of them becomes Trigens, like Jack says: "Looks like this bastards kill the islanders."