The various types of mutant creatures that inhabit the Wastelands were mostly created by radiation. This is where mantises, geckos, radscorpions, brahmin and the various mutant rodent species originated from. Radiation is also how ghouls - decrepit, zombie-like victims of massive radiation poisoning - are created. Many ghouls were once Vault dwellers living in Vault 12 under the city of Bakersfield (better known as the Necropolis after the War). As part of the vast Vault Experiment Program, the Vault 12 door was designed not to close properly. Thus, the Vault's inhabitants were exposed to massive amounts of radiation, leading to most of them becoming ghouls.
The What makes you S.P.E.C.I.A.L. video on Endurance makes mention that "Radiation Poisoning may have enlarged your favourite meals". Indeed, this "Radiation Gigantism" has been shown by many types of the Wasteland's creatures, such as Giant ants, Radroaches, Bloatflies, and Mirelurks.
Forced Evolutionary VirusEdit
The other major source of mutations in the Fallout world is FEV: the Forced Evolutionary Virus. The super mutants, floaters, centaurs and possibly wanamingos were products of FEV infection. Harold was also mutated by FEV, even though he looks like a ghoul.
It was initially called the Pan-Immunity Virion Project and was created to fight a disease called the New Plague. However, abnormal side effects were observed in test subjects. The test animals began to grow dramatically and their brain activity increased. Seeing the potential there, the Army sent its own people to secure the project, renamed it FEV: Forced Evolutionary Virus, and began performing experiments on humans within the Mariposa Military Base, which would turn soldiers into abnormally strong and intelligent supermen.
After the Great War, and the exodus of Roger Maxson and his troops to the Lost Hills Bunker, the base sat quiet until it was discovered by a group of explorers including Harold and Doctor Richard Grey, most of whom died when they encountered the automated defense systems. Soon, only Grey and Harold remained. Harold was knocked out (he never did find out how) and woke up out in the desert looking like a ghoul. Grey was knocked into a vat full of FEV by a crane and managed to crawl out. He began to mutate in horrible ways, turning into a sort of blobby kind of thing. He developed psychic powers, which were enhanced by consuming living minds to expand his own brainpower. Slowly, he started doing his own experiments with FEV.
Grey found that many of the test subjects were metamorphosing into giant, mindless brutes with the intelligence of a child, if they survived at all. However, a rare few test subjects became super mutants. Immune to disease and radiation and blessed with exceptional strength, intelligence and endurance, super mutants were superior to humans in every way. They also possessed the exceptionally long life of the mutated as well as total sterility. Grey --who now called himself the Master-- lamented the needless destruction of the war. He decided that he would have to force humanity to evolve. Those who could not evolve would die.
Creating super mutants was a very hit-or-miss process. The great majority of super mutants produced by the Master and later his Lieutenant in Mariposa's Vats were big, dumb brutes. Physically, they were vastly superior to humans, but they had the intelligence of children. What exactly causes some mutants to be brilliant and others to be stupid is unknown. As a rule of thumb, the less damaged or altered by radiation the subject's DNA, the better.
Aside from super mutants, the Master also created a variety of other creatures. Floaters are hostile mutant creatures that likely originated from the FEV-infected flatworms mentioned in the FEV experiment disk, and were later further enhanced by the Master's experiments. Centaurs were created by tossing a varied mix of dogs, cats, brahmin and other animals into a vat and seeing what came out the other end.
There are a few other types of mutant creatures in the Wastelands that don't directly stem from FEV or radiation. These are mostly the intelligent animals and one spore plant encountered in Fallout 2. The intelligent spore plant and radscorpion in Broken Hills were another, separate experiment conducted by a scientist there. The wanamingos encountered in Fallout 2, which were a pre-War experiment conducted by the U.S. military similar to the Enclave's intelligent Deathclaw research, were all, without exception, killed by the protagonist in Fallout 2. Occasionally, other intelligent animals have appeared here and there in the Fallout games, but all have been flukes. A race of intelligent raccoons living in an colony known as the Burrows was originally intended to appear in Fallout, but was cut due to style issues.
Frank Horrigan, the Enclave's most powerful enforcer, was produced by a series of treatments involving a carefully modified version of the FEV, surgery, a variety of other drugs and the addition of a suit of Power Armor. Lastly, there is Harold, who looks a lot like a ghoul, but actually is a different kind of mutant than the standard necrotic meta-humans. Ghouls are the result of massive radiation exposure, while Harold was mutated by brief FEV exposure without being dipped into one of the nutrient vats containing the virus. FEV's mutational results are at best unpredictable, depending on the subject's genetic quirks and amounts of exposure to the post-nuclear wasteland's ambient radioactivity, so one never can tell what the mutational result of an FEV infection will be (just look at the Master). Harold's existence was an oddity, the result of a unique combination of a specific degree of exposure to the virus, a specific degree of prior radiation exposure within his own body and any number of other factors. While FEV does not produce ghoul-like mutants like Harold as reliably as it does Super Mutants when humans are infected by it, it does occasionally spit out something that looks like one.
Another important exception is Talius, the ghoul-like creature that was hiding with the Followers of the Apocalypse and who was turned into his current state by FEV exposure, much like Harold, though under different circumstances.
There are human societies that have undergone gradual mutations over several generations due to either environmental conditions like exposure to the modified airborne strain of FEV that permeates the Wastelands of New California, radiation exposure, or other reasons. Two examples are the Slags (mutated by genetic adaption to life underground) and the Beastlords (mutated by a unique form of radiation). Additional forms of these such mutants include the Trogs of the Pitt, the Swampfolk of Point Lookout and the Tunnelers of Hopeville.
Over the years, different people have been in charge of Fallout canon and have come up with different interpretations on this subject. Mutant creatures and where they come from is one of the most controversial topics in Fallout canon and it's very easy to annoy veteran scholars of the setting by getting things wrong or even just partially right.
First, Chris Avellone seemed to prefer the "FEV explains everything" theory of mutation. He suggests that when the Glow was hit by the Chinese warheads, the tanks holding the virus burst and the FEV was vaporized and shot into the air. It was then mutated by the radiation from the blast, and went on to help radiation create the various mutant animals we know and love as well as ghouls. The problem with this theory is that first off it's a bit redundant since Fallout radiation is already fully capable of producing the mutations seen in the game's world. Second, we know from the Lieutenant's mention of inoculation from mutant FEV that any airborne strain of FEV could not cause these mutations. If it did, why didn't any humans mutate from exposure to the airborne FEV? Furthermore, if radscorpions, geckos, molerats, deathclaws and other creepie-crawlies are all products of FEV, how can they breed as much as they do? Remember, FEV causes sterility. Avellone eventually admitted that he was wrong, and that most mutations in the Fallout world were caused by the ambient radiation.
Next, Chris Taylor offers a different point of view on how ghouls are made. He suggests that when people with too much radiation damage are dipped, you get ghouls. When people with mild radiation damage are dipped, you get stupid super mutants. When people with little to no radiation damage are dipped, you get intelligent Super Mutants. This hypothesis would mean Harold and Talius are normal ghouls. However, since the population of Vault 12 was never dipped, not all ghouls could be made this way, only that FEV would speed up the process slightly.
Enclave Mutation Policy Edit
The Enclave holds the view that any "human" who has been "exposed" to the post-nuclear world's ambient environmental radiation is a mutant and that all mutants should be destroyed and disposed of. The Enclave's definition of "exposure" would seem to mean that only Enclave members and those people still residing in Vaults count as "pure" human beings.