"Mutos are the scarred relics of ourselves. Monsters created by the chemical weapons used in the first century of this war. They were banished into the wastelands where they live and scavenge like animals."
― Nyder, explaining about the Mutos. (Doctor Who, "Genesis of the Daleks")
Basil Karlo/Clayface (DC Comics) became a Deviant after an over-exposure to some chemicals, which mutated his entire cellular structure, turning him into a shape shifting monster of a clay-like substance.
While successfully regenerating his lost arm via Lizard DNA, Curtis Connors/Lizard (Marvel Comics) became a humanoid reptile in the process.
Deviant Mutates (Marvel Comics) are severely mutated Deviants with greater genetic instability than normal Deviants.
Henry Philip McCoy/Beast's (Marvel Comics) secondary mutation from experimentation gives him a humanoid feline appearance.
Peter Parker/Spider-Man (Marvel Comics) has become the Man-Spider.
While he used to be a normal Inhuman, Triton (Marvel Comics) gained an aquatic, monstrous appearance due to the Terrigen Mist.
After Rick had failed to restore humanity, people have become Cronenbergs (Rick and Morty) due to another faulty antidote being spilled over all populations.
Ninja Turtles (Teenage Mutant Ninja Turtles), were regular turtles before they came into contact with mutagen and mutated into anthropomorphic turtles.
Mutagen (Teenage Mutant Ninja Turtles), alien substance that is the key explanation for the mutations. When the exposed to the mutagen, the affected person may experience a change in form, an increase in both size and strength, and may even experience a development in special powers.
Takeshi/Tiger Claw (Teenage Mutant Ninja Turtles 2012 TV series) became mutated with tiger DNA, gaining the strength and claws of a tiger, nightvision, and a keen sense of hearing and smell.
Xever Montes/Fishface (Teenage Mutant Turtles 2012 TV series), touched a snakehead and came in contact with mutagen, gaining ability to breathe and swim underwater, powerful jaws and venomous fangs.
Chris Bradford/Akita Dogound (Teenage Mutant Ninja Turtles 2012 TV series), was bitten by the Shredder's Akita and came in contact with mutagen, gaining a keen sense of hearing and smell, and superhuman strength with his left arm…
… when he came in contact with a second dosage of mutagen, he mutated into the skeletal werewolf Rahzar. In his new form retains his strength, regains his speed and agility and possesses razor sharp claws that he can project.
After being kidnapped by Neo-Umbrella, Deborah Harper (Resident Evil 6), was forcibly injected with a mixture of different viruses and mutated into a spider-like monster.
After being killed by a gang of T-Abyss Virus Oozes, Rachel Foley (Resident Evil: Revelations), was revived by the T-Abyss Virus as an Ooze.
The G-virus (Resident Evil series) is a highly-advanced strain of the Progenitor Virus created by William Birkin. The virus possesses fierce-regenerative abilities that revitalize the body, but also mutates its host into a horrible monster.
William Birkin (Resident Evil 2) was the creator the G-virus. After being mortally wounded, he infected himself with the virus, becoming a mutated creature of incredible strength, but slowly began to lose his humanity afterwards…
…as William became increasingly injured during his fights with Leon S. Kennedy and Claire Redfield, the G-virus rapidly healed and evolved his body to that a clawed juggernaut…
…In his third form, the G-virus mutated William in a hulking, multi-armed brute with his humanity all but lost…
…At this point in his forth form, William lost any resemblance to that of a rational being, transforming into a ferocious bipedal monster…
…In his fifth and final form, William has sustained significant damage to the point where the G-virus was unable to fully heal him, leaving him to become nothing more than a mindless mass of flesh and teeth.
Albert Wesker (Resident Evil) after absorbing the Uroborous
Chimeras (S.T.A.L.K.E.R.: Call of Pripyat) are Mutants that both behave like and resemble Panthers, having great running speed and leaping.
While their origin is a mystery on how they exactly came to be, the Pseudogiants (S.T.A.L.K.E.R. series) are the most feared and most powerful Mutants within the zone, appearing as nothing more but bloated abdomens with limbs.
Being former Humans that have been exposed to radiation as well as the existing anomalies found throughout the zone, what they are supposed to be physiology wise, has been stripped away clean, turning them into monstrous Snorks (S.T.A.L.K.E.R. series). Beyond that however is their increased muscle power and reflexes.
Alexei Stukov (Starcraft), the only infested terran to fully retain his original personality.
The Avian Mutants (System Shock) used to be normal infant Lizards inside eggs that had been affected by reverse-breeding the reptiles with the use of the Squamata order, combined with the use of exposing them to UV radiation.
Humanoid Mutants (System Shock) are the result of Humans being exposed to mutagenic viruses. Even though they are weak, they can be dangerous if there was a horde of them.
Plant Mutants (System Shock) used to be regular plants that have been exposed to viral mutagens within the groves of citadel station.
Created by S.H.O.D.A.N.’s V-5 Mutagen Virus, the Virus Mutants (System Shock) are a tad more powerful than the Humanoid Mutants, and are presumably former Humans as well.
Zero-Grav Mutants (System Shock) are creatures that were formally cell-like organisms that have been mutated into a multi-cellular form to the point of being medium-sized to Humans.
After the Hybrids have been mutated further due to the Grubs having mutagenic properties, they turn into Rumblers (System Shock 2) which are faster, tougher, and stronger.
Makonoids (Final Fantasy VII) are former humans who were heavily mutated by exposure to concentrated Mako energy.