Freeza (Dragon Ball) is a mutant amongst his species, a trait he inherited from his father, which accounts to their abnormally high power level and cruelty.
Utsuro (Gintama) is a mutated human born under the effects of Earth's Altana, remaining immortal so long as he remains on the planet to draw from the source.
Kouka (Gintama) is a mutated Yato born under the effects of Kouan's Altana, remaining immortal so long as she remains on the planet to draw from the source.
Spider-Man (Marvel) has become the Man-Spider.
Jūgo (Naruto) in the process of mutating himself via his enzymes.
Sasuke Uchiha (Naruto) mutating his own body with the Cursed Seal of Heaven.
Kimimaro (Naruto) mutating his own body with the Cursed Seal of Earth.
Orochimaru's (Naruto) Test subjects
White Zetsu (Naruto) are humans who are mutated in a degrading manner from the "God: Nativity of a World of Trees".
After being kidnapped by Neo-Umbrella, Deborah Harper (Resident Evil series), 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 series), 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
Alexei Stukov (Starcraft), the only infested terran to fully retain his original personality.
Chaos (Sonic the Hedgehog) was once a normal Chao, but was mutated into his current form by prolonged exposure to Chaos Energy.
Ninja Turtles (Teenage Mutant Ninja Turtles) were normal turtles before they came into contact with mutagen and mutated into anthropomorphic turtles.
Mutagen is a alien substance in the TMNT franchise that serves as the key explaination for the mutation of the Ninja Turtles, Splinter, and most of the foes they fight. 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.
When Xever Montes (Teenage Mutant Turtles 2012) touched a snakehead and came in contact with mutagen, he became mutated into giant large-fanged snakehead Fishface. In his mutated form, Fishface is able to breathe and swin underwater, and possesses powerful jaw strength and vemonous fangs.
When Chris Bradford (Teenage Mutant Ninja Turtles 2012) was bitten by the Shredder's Akita and came in contact with mutagen, he became mutant into the giant Akita Dogound. In his mutated form, Dogpound posseses a keen sense of hearing and smell, and superhuman strength with his left arm...
...When Chris came in contact with a second dosage of mutagen, he became mutated into the skeletal werewolf Rahzar. In his new form Rahazar retains his strength from Dogpound and regains his speed and agility. He also possesses razor sharp claws that he can also project.
When Takeshi (Teenage Mutant Ninja Turtles 2012) was mutated by the Kraang when he was kid, he became mutated with tiger DNA, becoming vicious Tiger Claw. In his mutated form he possesses the strength and claws of a tiger, nightvision, and a keen sense of hearing and smell.
Half of Gido's (Marchen Awakens Romance) was horrifically mutated by Chimera in order to punish her boyfriend Ian for disobeying orders.