When Osmosians (Ben 10) attempt to absorb too many life forms at once, their own DNA will be overwritten with alien DNA, causing them to become a chimeric mixture of those creatures.
Kevin 11,000 (Ben 10) in his mutation form.
Devlin Levin (Ben 10) in his mutation form.
Kain as Scion of Balance has the powers of Nature Guardian that: Can warp creatures growth and abilities,"Creating new and weird life forms".
A Xenocyte (Ben 10: Alien Force/Ultimate Alien) in the process of mutating a human host into a DNAlien.
Diva's (Blood+) singing can cause any human infected with the Delta 67 agent to mutate into a chiropteran.
When his body began to mutate as a result of its instability, Fusion Zamasu (Dragon Ball Super) called upon the Light of Justice...
...to spread said mutations to his right arm.
Dark eco (Jak and Daxter series) is a dark, poisonous substance that can infect and mutate any life form or object it comes in contact with.
Diego Brando (JoJo's Bizarre Adventure) uses his stand, Scary Monsters to transform Gyro into a dinosaur.
The mutant Infectia (Marvel Comics) utilized her ability to mutate living organisims for criminal self gain.
Orochimaru (Naruto) can force his subordinates and subjects into mutation via a Cursed Seal.
Mitochondria Eve (Parasite Eve) awakens the mitochondria in the cells of any life form she comes in contact with, turning them into horrific Neo-Mitochondrial Creatures.
Chemical X (The Powerpuff Girls)
Yume Hasegawa (Pupa) fed her brother Utsutsu some of her flesh to transform him into a monster just like her.
Christine "Four-Eyes" Yamata (Resident Evil: Operation Raccoon City) can use her knowledge of virology and a special gun to either infect humans with the T-virus or mutate zombies into Crimson Heads.
Even though he is infected of NE-α Type parasites, Nemesis-T Type (Resident Evil 3 2020) can also mutate present Zombies by physical contact whilst one of the parasites are channeled through either of his hands.
Oozesquitos (Rise of the Teenage Mutant Ninja Turtles) can mutate humans via their sting.
SCP-040 - Evolution's Child (SCP Foundation) can manipulate living matter allowing her to modify existing life forms to create new ones. Her creations include a shape shifting sweater, a gasbag that plays tunes, and a fuzzy wuzzy wall crawling critter.
SCP-427 - Lovecraftian Locket (SCP Foundation) is a locket capable of repairing cellular damage and purging any invading compounds or infections when it is exposed to biological material,...
...but will rapidly mutate organisms into shapeless masses of tissue nicknamed Flesh Beasts if it's used for extended periods.
The energy from Dark Gaia (Sonic the Hedgehog) mutated Sonic the Hedgehog into a werehog.
Erazor Djinn (Sonic the Hedgehog) transformed into Alf-Layla-wa-Layla, what Sonic described as an "incomplete monster", when he absorbed four of the seven World Rings.
The Corrupting Light (Steven Universe) was a weapon utilized by the Great Diamond Authority to corrupt and mutate the remaining Gems on Earth.
When exposed to the mutagen (Teenage Mutant Ninja Turtles), 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.
Sh'Okanabo (Teenage Mutant Ninja Turtles 2003 TV series)
Lords of Change (Warhammer 40k) can mutate any Chaos mortal followers into grotesque mutants.