The Mindless Ones (Marvel Comics) truly live up to their name.
Teon (Marvel) is so detached from his humanity that he is little more than a tamed dog.
The Atrocity's (Spider-Man: Edge of Time) only coherent thought is getting revenge on Spider-Man for his perceived role in its creation.
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 become hyperaggressive 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 Comi) manifests and takes over Sonic's body, he is hyperaggressive, violent, and a danger to friend and foe alike.
While in the form of Great Apes, (Dragon Ball) Saiyans 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.
After extensive damage and mutation, Nemesis (Resident Evil) loses track of his programmed objectives and becomes a mindless 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.
Chopper (One Piece) has a transformation which 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 (Gin Tama) goes insanely feral after being taken over from his fusion with Benizakura.
Beast Rune representing animalistic rage. (Suikoden 2)
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 (Pokemon Mystery Dungeon)
Kid Buu's (Dragon Ball Z) only coherent thought is to cause as much destruction and death as possible.
Okumura Rin (Ao no 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.