Grimlock has had several of these in Transformers Meta and will probably have several more.
He raged against Barricade and Soundwave in the third episode when they were holding Bumblebee captive.
He beat the heck out of Barricade in the second episode when Barricade nearly took out Bumblebee.
Furthermore, he charged furiously at Slipstream when Bumblebee was, yet again, being held captive.
In the same episode, he went absolutely crazy when Bumblebee had been kidnapped and went so far as to beat up a corpse because there was no one else around that he could kill.
In the second episode, he tried to snap Wheeljack in half when the latter left Evac and Bumblebee by themselves in a potentially hostile environment.
Inevitably, Grimlock's Unstoppable Rages are commonly caused by, but are not limited to, something happening to Bumblebee.
In a My Little Pony / The Dresden Files crossover fic My Little Denarians, the ponies get exposed to much more non-cartoon violence than Harry would have liked - and they learn fast. At one point Derpy is shot by a sniper, causing Dash to charge across the street at the attacker. She may be a cartoon pony but she is a supersonic cartoon pony - after running across the street and up the stairs, Harry finds the room splattered with blood and bits of flesh and the furious Dash Pummeling the Corpse
Kyon: Big Damn Hero. If you do anything to harm the SOS Brigade, Kyon - or if you hurt him, Haruhi - will lose it and you will suffer.
What About Witch Queen? has Friedrich, king of the Southern Isles, whose Hair-Trigger Temper manifests itself like this. It's not necessarily good quality in a ruler or a father, as he's Not Himself when in this state and must lock himself in his room to prevent himself from hurting anybody.
Megami No Hanabira: Yuuna flips her lid when a demon injures her girlfriend Nanami: she tears through them nigh-effortlessly with her own demons, before finally laying the last one out with a single punch.
In the Firefly fic Forward, Jayne flies into one of these when River takes a bullet for him during the assault by Niska's goons. It is described as Jayne "seeing red." Jayne periodically has these moments throughout the story, when his anger overrides good sense. Later on, River goes into one of these rages when a Hand of Blue shoots Simon, and in the resulting scene reduces his face to hamburger by repeatedly stabbing it in a hysterical, savage frenzy.
Logan Wright from Dalton has some rather serious anger issues, causing him to violently lash out at virtually everyone on a semi-regular basis.
Blaze from The Legend of Spyro: A New Dawn enters this when the curse preventing him from helping his daughter fight the Warden is broken. He shoots the Warden a Death Glare, plows straight through everything between him and his target (including the cage he was in and a mob of Gargoyles), and sends the Warden flying clear across the room twice in a row. He then leads the former slaves and his daughter in a No-Holds-Barred Beatdown on the Warden, ending it by blasting the Warden through a wall. Do not mess with Blaze's daughter!
Sleepwalker will explode into this trope when confronted with his Arch-Enemy Cobweb, who trapped him on Earth, or his Evil Counterpart Psyko, who murdered the parents of his Heroic Host Rick Sheridan. Sleepwalker's anger typically pushes him beyond his normal limits when dealing with Psyko, although even then the result is a bloodbath from which Sleepwalker typically emerges more dead than alive. An unusual twist on this trope is that Cobweb has also played on Sleepwalker's anger to manipulate him into losing focus and making bad decisions that would get him killed...at least until Rick serves as a Spanner in the Works and sets Sleepy straight.
When Violet discovers Ludlow's true motives in Episode 4, she basically snaps. And spends the remainder of the fic turning this trope Up to Eleven, which is more than enough to tip the balance of power in the protagonists' favor.
In The Wizard in the Shadows, these happen to several characters, most frequently Harry. The usual sign of Harry having been in Unstoppable Rage mode is a large crater. One caused by a previous incident of Rape Is a Special Kind of Evil was one hundred yards across. The size of a soccer pitch, in other words.
Harry when he finds out that Saruman has been torturing Sirius for the best part of seven years. He throws Wormtongue from the top of Isengard, catches, him, gives him to Éomer, drags Saruman down into the dungeons by his beard(after letting Emrys break his nose and forcing it back into joint), vaporizing the doors of Isengard, and when he finds Sirius he loses his temper completely and repeatedly uses the Cruciatus curse.
Also, Emrys doing his level best to drown Wormtongue after finding out that Wormtongue raped his sister. Theodred has to call him off, much like the Batman example above.
Harry when Ginny is poisoned by the Witch-King, unleashing the full power of a Patronus infused thunder storm. Part of the weakest bolt vaporises a Mûmak.
Ginny when Harry dies unleashes three enormous tornadoes of fire on the army of Mordor, destroying the physical forms of three Nazgûl in the time it takes to snap your fingers.
Pages Of Harmony involves Fluttershy going into this at one point. She's had Kindness stripped from her so completely that she hallucinates that her pets are the ponies she believed wronged her, and tears them mercilessly to pieces. She then turns this attack on Twilight, whose Well-Intentioned Extremist actions were the cause of her present suffering, and they pummel one another into a bloody mess, with Fluttershy tearing chunks off of Twilight's skin before Twilight eventually manages to win by a margin.
In the RWBY fic Massages, Team CDRL beats up Jaune as usual, but when they reveal that they plan to rape every girl in his Unwanted Harem, he loses it and gives them a No-Holds-Barred Beatdown. The others are forced to knock him out, not out of concern for Team CRDL, because he was about to burn himself out from releasing so much energy at once.
'No fighting policy' be damned. Morals and the law didn't mean anything anymore. Jaune would kill these motherfuckers.
In Mega Man Reawakened, Robert enters one if someone he cares about or has come to respect is hurt.
Shepard's "fight" with Morinth in Avatar Of Victory begins when the latter, trying to put Shepard under mind control, targets a memory of Shepard's Love Interest Liara. Shepard flips out, enters the Avatar State, and beats Morinth into a bloody pulp before spacing her, all while declaring that this is what happens to anyone who threatens the Avatar's loved ones.
In the VERY darkMy Little Pony fanfic The Night That Never Ended this happens with Rainbow Dash when she sees one of Nightmare Moon's minions brutally murder several ponies helping her. She attacks the guard, and the scene cuts away... ...And when the scene cuts back, Rainbow Dash has not only beaten the pony to death, but has been Pummeling the Corpse for quite some time.
When the Tutanshaman Brothers threaten Draggy by saying that Draggy will be very dead after they're done dealing with Guile, this enrages Guile so much that he lets loose a furious roar which diffuses a streaming blast of fire, causes a tremor, cracks the ground and the tree trunks around him, and blows off the leaves on the trees and hedge nearby.
The Soul Eater oneshot They Shook Death City centers around Maka's quite frankly terrifying rage against people she deems bad drivers. Poor Soul spends most of the story fearing for his life in the passenger seat.
RWBY: Reckoning has Darrel being pushed to this after Ruby is shot, Blake is hurt, Sun is insulted, and he's used as a Human Shield, courtesy of Roman. Having had enough of his friends being hurt, he finally cuts loose, throwing him around like a rag doll. Of course, Roman being Roman, he fights dirty.
This happens to Rainbow Dash in Sunset of Time when Sunset Shimmer kidnaps and nearly kills Fluttershy. She leaps into the air and flies at her, ("supreme fury" on her face) screaming that she's going to kill her... ...But Sunset Shimmer just shoots her down with magic, ending her rampage before she can do anything.
In Gensokyo 20XXV, this is Reimu's response to Yume Ni kicking her in the face, the which of she felt, and she couldn't be stopped until she stabbed the latter with a pair of scissors.
Mamizou: Swinging them around madly like mad warrior and his sword, she came after her and, no matter the effort, she could not be stopped.
Pete Malloy in the Adam-12 fic "Into The Forest". One of the guys holding him and his girlfriend prisoner tries to rape her, a very bad idea. Pete breaks the strap he's tied with, despite being injured, and charges the guy with every intention of killing if need be.
Happens to Roy Desoto in the Emergency! fic "Return". He sees John Gage's uncle trying to take off John's pants to rape him, and though it is not a slash story, they are close friends, and Roy lets the uncle have it. Similarly, in the author's fic "Revenge," a kidnapper from whom Roy rescued John comes after them and catches up to them at the station. Enraged at how they treated John, Roy gets physical with the guy.
Friendship Is Magical Girls: If members of the Emerald Flame Dragon clan allow themselves to lose control of their anger, they can fall into one of these, symbolized by their magic flames shifting from green to red. Spike's master's father fell into one of these long ago when a member of the rival Soaring Lion Eagle clan killed his fiance; the result was the near-extinction of the Soaring Lion Eagles. Spike himself falls into this when Snips shoots Twilight, leading to him hunting Snips down and killing him in cold blood.
Sailor Moon: Legends of Lightstorm: At the climax of her debut episode, Sailor Jupiter uses this to transform through sheer force of will alone, something no Sailor Scout has ever done before.
Betelgeuse has a reputation for this among other ghosts in the Danny Phantom/Beetlejuice crossover Say It Thrice. They know he's really angry about the fact that he's been separated from his "Mortal Bride," and they're doing their best to keep out of his way. Those few who are foolish enough to try him in his present state end up maimed. And when he finds her and discovers that another poltergeist is about to rape and murder her, there is no word strong enough to describe the fury which results.
In three stories by Caro June, Imperial guards find out how bad an idea it is to rough up Leia right in front of Han. In Going Solo, one of the guards decides to 'distract' Han by smacking Leia. In The Way It Should Be, one of the guards makes clear his intent to rape Leia. Both scenes have the same result: Han rushing them in blind fury, despite being severely injured, grabbing their necks in a vice grip that probably rivals Vader's. He would likely have killed the attacker both times, but in both stories, he's cold cocked a second later. In her story The Princess And The Pirate, Han goes wild twice because of someone harming Leia. The first time, a guard whacks her when she trips, and Han headbutts him hard before getting, yup, knocked out. The second time, she's hit with a heavy stun, and Han reacts with wild rage, struggling against his captors as hard as he can. It doesn't work though, he still gets dragged off and beaten.
Qui Gon Jinn gets into one twice in The Marionettes Dance trying to protect his unconscious apprentice Obi Wan. The first time, he slices various limbs off four guards after having used a Jedi Mind Trick to fool them into ignoring his lightsaber. Later, he's really set off when they lie and say Obi Wan died. He crushes one guard's throat but doesn't kill him, then does kill two more with his bare hands. The first guy gets a piece of nasal septum driven into his brain by Qui Gon's elbow slamming into his face,and the second guy gets his neck snapped after Qui Gon choke holds him and forces him to admit that Obi Wan is still alive and where he's at. He may have felt he had no choice but to kill that one, he was wielding a box that controlled shock collars that both Jedi had been forced into wearing, and the Electric Torture was really ticking off Qui Gon already.
Mara Jade is raped in The Unthinkable, and Luke nearly goes all the way into The Dark Side mode, knocking out two Barabels without much thought and then viciously Force-choking her attacker. Mara and Han beg him not to kill the guy, and Han is ready to stun Luke with his blaster if he doesn't stop. Mara tells him afterward that she was scared seeing him like that. He does reign in his anger and let the guy live, but snarls that he WILL kill the guy if he ever touches Mara again. Luke himself is saddened by it when they talk at the hospital and thanks Mara for stopping him.
Both Han and Leia's Berserk Button gets pushed in Welcome Back Your Dark Memories. Han and Leia are in a tapecafe and a guy from Jabba's Palace happens to be there. He makes lewd remarks about Leia, and Han gets into a knock-down drag-out fight with the guy. They separate, and Leia moves in to tell Han it isn't worth it. But then, the guy breaks out a blaster to shoot Leia and makes another remark, and she pulls a Vader, blocking the guy's shot with the Force, then Force-choking him until Han has to tell her it isn't worth it to kill him. She does let him live, but only after a few choice remarks about respecting women.
Asuka and Unit 02 continued to rage, a dam within having burst. Everything has to die! Die for your sins! Die! Die like momma died! Die and leave me alone! Always alone! Die!
The Second Try: Subverted. When Gendo used the name of Aki to taunt Asuka she got raving mad. She broke free despite of being handcuffed and surrounded by Gendo’s agents, leapt at his desk and tried to throttle him. However they managed to sedate her before she closed her hands around his neck.
In the Worm fic Quicken, it happens to Emma several times in the first story arc. She runs into several gang squads, and they threaten her until she goes nuts and kills them all.