The user is or has the ability to take on the form of a monster, a creature that is often hideous and may produce fear or physical harm by either its appearance or its actions. The word "monster" derives from Latin monstrum, an aberrant occurrence, usually biological, that was taken as a sign that something was wrong within the natural order.
The word connotes something wrong or evil; a monster is generally morally objectionable, physically or psychologically hideous, and/or a freak of nature.
Kaguya Ōtsutsuki (Naruto) turning into a monstrous form when she lost control of her powers.
Tony Tony Chopper (One Piece) in his Monster Point form.
Due to being experimented on with Dark Eco for two years, Jak (Jak and Daxter) can transform into "Dark Jak".
Kevin Levin (Ben 10) often takes on the form of a monstrous, chimeric fusion of aliens when he absorbs energy from the Omnitrix/Ultimatrix.
Joseph Bertrand III (Infamous) in the form of the Behemoth.
Zarbon (Dragon Ball Z) in his monster form.
With all seven Chaos Emeralds, Chaos (Sonic the Hedgehog) becomes the leviathan-like Perfect Chaos.
Sonic the Hedgehog (Sonic the Hedgehog) unwillingly transforms into a "Werehog" every sundown due to the influence of Dark Gaia.
Eclipse the Darkling (Archie's Sonic the Hedgehog) using Power Up: Monster Form.
Sasuke Uchiha (Naruto) when he activates the Cursed Seal of Heaven.
Kimimaro when he activates the Cursed Seal of Earth.
Jūgo (Naruto) transforming
A mass amount of Cursed Seal users going berserk.
Tosen (Bleach) releasing his Resurreccion, Grillar Grillo, to transform into a gigantic cricket Hollow.
Sora (Kingdom Hearts) in his Anti-Form.
Daxter (Jak and Daxter) can transform into "Dark Daxter" due to further exposure to Dark Eco.
Richard Wilkins III (Buffy the Vampire Slayer) in the form of the snake demon Olvikan.
Ninja Turtles (Teenage Mutant Ninja Turtles; 1987 TV series) in their super mutant forms.
Carter (Teenage Mutant Ninja Turtle; 1987 TV series) can transform into a yellow skinned creature with super-strength due to exposure to Mutagen.
Xion (Bloody Roar) in the form of the Unborn.
The monstrous Giga Bowser (Super Smash Bros.)
Ganondorf Dragmire (The Legend of Zelda) in his beast form, Ganon.
Vincent Valentine (Compilation of Final Fantasy VII) in the form of Chaos.
Saiyans (Dragon Ball) transform into feral Great Apes while under the influence of a full moon.
Goku (Dragonball Evolution) transforms into the monstrous Oozaru under a solar eclipse.
Sonson (Marvel vs. Capcom 2) can temporarily take on a more monstrous monkey form.
Cadence Nash (The Troop) is a human/Blood Thrasher hybrid, and can assume her Blood Thrasher form at will.
Kaine (Marvel Comics) as the monstrous Tarantula.
The SA-X (Metroid Fusion) in its monster form.
Mohinder (Heroes) slowly transforms into an insect monster.
Elfman strauss (Fairy tail) takes on a monstrous form
Gear REX (Metal Gear Solid: Peace Walker)
Bedaxtra (Dexter's laboratory)
Monsters (Monsters, Inc.)
Calvin Dion Avericci (Archie's Weird Mysteries) is a shopaholic so greedy, he sold his soul to keep buying, resulting in his demonification.
As a Lummox, Riki (Saga Frontier) has the ability to transform into and use the traits of any and all monsters.
Jack-Jack Parr (The Incredibles)
Letting the Ghost ÄRM, Hydra erase whatever humanity was left in her heart, Chimera (Marchen Awakens Romance) succeeded in obtaining the ÄRM’s power. Deprived of human feelings and sharing Hydra’s horrific body, Chimera had truly become a monster.