In his Bankai form, Kenpachi Zaraki (Bleach) becomes a mindless, feral berserker unable to differentiate between friend and foe and fighting without any regard for his own safety.
Thor (Marvel Comics) in Warrior's Madness which boosts his strength.
The Mindless Ones (Marvel Comics) truly live up to their name.
James "Logan" Howlett/Wolverine (Marvel Comics) is prone to entering a "berserker rage" when in close combat, during which time he lashes out with the intensity and aggression of an enraged animal.
Teon (Marvel Comics) is so detached from his humanity that he is little more than a primal beast.
The Atrocity's (Spider-Man: Edge of Time) only coherent thought is getting revenge on Spider-Man for his perceived role in its creation.
X-24 (Logan) is a being of barely-controlled rage.
After being grafted to experimental technology, Brick Baxter/Cyber Shadow (The Young Guardians) enters a mindset of feral hatred, anger, and insanity.
Though friendly and docile by day, by night, Remlits (The Legend of Zelda: Skyward Sword) become hyper aggressive and violent due to Batreaux's aura cursing Skyloft.
Peyton Westlake (Darkman)
The Heartless (Kingdom Hearts) are mindless and function on instinct.
When overdosing on Venom, Bane (DC Comics) enters a mindset of berserk rage.
After losing his memory of the last six months of his life, Superboy (Young Justice), having not even existed six months ago, is reduced to a mindless animal driven by basic instincts.
The Turok-Han (Buffy the Vampire Slayer) are ancient and powerful vampires who are as feral and single-minded as animals.
Iblis (Sonic the Hedgehog) is the raw power of Solaris, who's only coherent thoughts are wreaking havoc and causing destruction.
When faced with life-threatening situations and deprived of his sword, Inuyasha (InuYasha) is taken over by his demonic blood, increasing all of his physical abilities, but leaving him in a berserker rage in which he cannot tell friend from foe.
When Super Sonic (Sonic the Comic) manifests and takes over Sonic's body, he is hyper aggressive, violent, and a danger to friend and foe alike.
While in the form of Great Apes, Saiyans (Dragon Ball series) are typically mindless and destructive.
Saïx (Kingdom Hearts) enters a berserker rage whenever he is under a full moon.
The SA-X (Metroid Fusion) in its monster form.
The Viera (Final Fantasy) enter a frenzy when exposed to magical mist, which grants them enough strength to break metal chains and kill armored humans with their bare hands.
Slash (Teenage Mutant Ninja Turtles)
The Heartless (Kingdom Hearts) are limited to functioning on instinct.
Spike (Buffy the Vampire Slayer)
After witnessing the destruction of an entire village, Asura (Asura's Wrath) enters an incredibly powerful berserker state.
While the Nemesis-T Type (Resident Evil) is initially surprisingly cunning and competent...
...after extensive damage and mutations it loses track of its programmed objectives and becomes a mindless feral beast.
Seras Victoria (Hellsing) can enter a berserker state when under extreme stress or pain...
..Seras in the midst of a berserker rage.
Saya Otonashi (Blood+) is prone to entering a berserker state through various means, such as being prematurely awakened from hibernation...
..Saya in the midst of a berserker rage.
Tony Tony Chopper's (One Piece) Monster Point transformation makes him incredibly powerful but loses all sense and rationality. After the timeskip Chopper no longer loses his mind while in this form.
Nizō Okada (Gintama) goes insanely feral after being taken over from his fusion with Benizakura.
Kagura (Gintama) losing control to her Yato instincts, becoming a berserk fighter.
Kamui (Gintama) losing control to his Yato instincts, becoming a berserk fighter.
Utsutsu Hasegawa (Pupa) losing control to his Pupa instincts after learning what his "sister" really is, becoming a berserk monster.
Beast Rune (Suikoden 2) represents animalistic rage.
When angered, Tony (Misfits) enters a feral rage who kills anyone he comes across and grants him enhanced strength/durability.
Stitch (Lilo & Stitch), was originally programmed with the singular instinct to destroy everything he touches and nothing more.
Primal Dialga (Pokémon Mystery Dungeon)
Kid Buu's (Dragon Ball Z) only coherent thought is to cause as much destruction and death as possible.
Rin Okumura (Blue Exorcist) when his sword, Kurikara is damaged and cracked.
On Pylea, Angel (Buffy the Vampire Slayer) was briefly transformed into a savage, berserk Van-Tal.
In his more monstrous form, Bloodscream (Marvel Comics) is little more than an animal.
In his aptly-named Runaway form, Mira (Dragon Ball) is consumed with intense rage and goes berserk.
To obtain the power of the Ghost ÄRM, Hydra, Chimera (Marchen Awakens Romance) allowed the ÄRM to erase her human emotions and memories. Once the transformation was complete, she had obtained the madness and behavior of the beast she fused with.
The Mark of Cain (Supernatural) subjects the bearer to murderous rage so intense that they simply must kill something or die.
PX-41 (Despicable Me 2) is a serum capable of turning any living thing into a mindless, indestructible monster.
Raphael (Teenage Mutant Ninja Turtles) goes into a berserker rage while fighting the Foot Clan.
In his past life as Takeshi Tatsuo, the Shredder (Teenage Mutant Ninja Turtles IDW) harnessed a berserker rage that enabled him to slaughter an army of five thousand samurai by himself.
Broly (Dragon Ball Super) is able to enter a state where he takes upon the strength of Great Apes while still in his human form, but loses all sense of self in the process.
Using Rage, Jonathan Reid (Vampyr) unleashes the beast within, losing control and striking with an unfettered fury.
In his Madness form, Venom (Marvel Comics) becomes powerful enough to match the strength of the Juggernaut, but also more ruthless and psychotic.
Feral Chaos (Dissidia 012 Final Fantasy) truly lives up to his name.
When severely beaten, Atsushi Kumamoto (Solo Leveling) enters a state of enraged frenzy, becoming the complete opposite of his normal self, attacking wildly with no sense of fear or self-control.
The Peacock Family (The X-Files) were degenerated mutants who could fly into a superhuman frenzy of animalistic rage, described as "going caveman" - sufficient to easily bludgeon a full-grown man to death with ease.
Berserker Servants (Fate series) are possessed of incredible power, which comes at the cost of the loss of their higher mental faculties as a result of the Madness Enhancement.
Beast-Man's (He-Man and the Masters of the Universe) animalistic rage matches with the beasts & monsters he dominates.