Human beings in general. Present your average person, chosen at random, with a credible threat to that which they care about, be the threat real or imagined, and chances are good they could become a raging berserker. There's a reason nearly all of the examples under Mama Bear and Papa Wolf are humans.
The same applies to politicians. They may appear to be friendly people, but some of them might have some skeletons in the closet.
Just as much as humans, dogs and wolves in particular fit this trope. Almost all canines have enough of a social order that, under the right circumstances, even the most territorial dogs and wolves can usually be approached by a human. But even the most friendly and tame canine you have ever seen is probably perfectly willing (and most are surpisingly capable- see their entry under Badass Adorable) to rip you limb from limb if they catch you trying to harm anyone they care about. Of course, there are those that avert or subvert this trope by being perpetually timid or just tearing you apart for the heck of it, these are not the majority.
Bernese Mountain Dogs deserve a special mention. Largefamily dogs who are very friendly, very sociable, good with children, and if raised around other animals (including cats and chickens, animals dogs would usually attack) they will treat those animals like members of the family. They form very strong bonds with their families...with makes them very protective over them. You threaten their families - especially the children - you're gonna find out just how well they can use their size against you. And even then, since they don't like violence other than the occasional play-fight, they'll mostly try and scare you off by using their very loud and very scary bark as a warning. Anyone stupid enough not to heed this warning deserves everything they get.
Casey Heynes: Mild mannered, kind and polite boy at an Australian school, bullied all his life by jackoffs who think that punching, slapping and taunting "the fat kid" makes you more "manly." This dumb bully won't be doing so again.
Christian Bale is a pretty nice guy and pretty relaxed but even he can lose it as those who've seen his infamous rant can attest too. Michael Caine, his co-star in The Dark Knight Saga, was utterly surprised when hearing about the outburst.
Skunks are kinda cute. Do not, under any circumstances, scare a wild one.
An animal example: The Gaboon Viper, one of the deadliest snakes, is very docile and easygoing, especially when compared to its feisty, aggressive rattlesnake relatives. Zoologists have reported that when picked up the snakes usually will not try to bite. However, as always, you should never push it too far. Those two-inch fangs can and will kill you, and their chubby appearance is made up of pure muscle. Did we mention that their venom will cause you to bleed out through youreyes, mouth, nose, and various other orifices?
The Semai of Malaysia are an anthropological rarity and a good example of this trope. They are a society with almost no interpersonal violence, with words for warfare not even existing in their language. During a communist insurrection in Malaysia, some Semai were involved in the fighting and they became effective fighters, almost fanatical ones. The Semai who fought described themselves afterwards as "drunk on blood," and they surprised themselves by their behavior, although they weren't upset by it.
C. S. Lewis once pointed out that the ideal Medieval knight was "Fierce in the field and meek in the hall." Of course, it really didn't come out that way, but that's why we call them ideals.
The Shaolin Monks are a prime example of this trope; they were peaceful till a group of bandits pushed them to the edge. Then they developed their own weapons and brand of Kung Fu- elegant, graceful, generally defensive- and also able to paralyze, maim, and kill in some of the most horrifyingly gruesome ways imaginable.
Reinhard Heydrich was born to artistic parents, and was by all accounts a shy, sensitive child. His mildly Semitic features did not go down well in early-20th century Germany, and he was frequently bullied as a child. From young adulthood onwards he transformed himself into a dashing naval officer and womanizer, and later climbed the ranks of Hitler's SS, becoming the kind of Nazi official that Nazi officials were frightened of. The general competence of "the blond beast" gave the Allies nightmares that he might succeed Hitler. Upon his assassination in 1942 the world sighed with relief; all except for the populations of the Czech villages of Lidice and Lezaky, who were murdered in reprisal. "The towns were burned and the ruins leveled."
Heydrich's former Navy commander, admiral Wilhelm Canaris. He was a rather frail, mild-mannered, soft-spoken man, caring father and fond of dogs and music. He was also the head of German military intelligence. During WWI he managed to escape from a prison camp in Chile, traveled to Buenos Aires mostly on foot and went back to Germany to start a career as a spy. He's known to have bypassed the clauses concerning submarines in the Treaty of Versailles by asking the Japanese to build them and then sell them to the German fleet. Well done. During WWII, he tricked Heydrich, Himmler AND Hitler into believing he was supporting them (while doing the exact opposite), and was at the very least aware of all plots against Hitler, and lent a hand in some. It didn't end well for him. Had a Crowning Moment of Funny, lighting his cigars with Hitler's orders.
The Bundeswehr, and most of their people are rather friendly, but put them in uniform and they go from wouldn't run a red light to crazy good soldiers.
Leonard Cheshire. A mild-looking slim and bookish young man who trained to become a bomber pilot in the RAF. After two full tours of duty in bombers (already beating the odds) he commands the elite bomber 617 squadron post-Dambusters raid. Using new methods and equipment and with his aircraft specially modified, his small unit destroyed with 12,000 and 22,000lb bombs many pinpoint hardened targets in Occupied Europe, including V2 launch complexes, the Battleship Tirpitz, concrete U-Boat pens, railway and canal viaducts, tunnels and bridges, and then developed methods to precision-mark targets from low level during raids. Eventually, he was flying a small single-engined Mustang fighter into a target in the moments before the attack began, putting marker flares precisely on the pin-point target. Awarded the Victoria Cross for continuous extraordinary leadership and success, he devoted the rest of his life to running the charity The Cheshire Homes; providing housing and help for the disadvantaged and disabled. Not really 'one who went berserk', but a man whose methods are now a normal feature of modern warfare with 'smart' precision-attack weapons, and who got back in his box when no longer needed.
Jimmy Stewart was largely considered to be one of the nicest men in Hollywood in particular and the world in general. He also joined the Army Air Force during the Second World War and fought as hard as he could to be placed with a combat squadron (his papers were repeatedly shelved because various higher-ups wanted to avoid the possibility of him dying in combat). He flew numerous missions over enemy territory as a common pilot, and while his official total is 20 missions, many others were uncounted. During the Vietnam War, he flew a combat mission aboard a B-52 as an observer, and finished his military career as a Brigadier General in the Air Force Reserve. Not bad for George Bailey.
Google: A company with the motto, "Don't Be Evil," has a laid back office atmosphere, entertaining April Fools jokes, and has access to most of the information on the planet? It is no wonder that you have some people thinking that they could take over the world in just a few mouse clicks.
Even worse, they also have access to top secret-files of several governments around the world. Yeah.... do not fuck with Google.
If you didn't know of him prior to meeting him, you might think that Simo Häyhä was an amiable but quiet man who seemed to have suffered an unfortunate facial injury in his past. However, bear in mind that he was a Finnish sniper in World War 2, is in the running for "single most lethal man to ever exist", and had a body count somewhere in the area of 700 kills. When Russia invaded Finland, Häyhä was a farmer who had already served his mandatory year of military service. The ensuing Winter War saw him cause so many Russian casualties that entire counter-sniper teams and whole artillery barrages were deployed to try and stop him...to no appreciable effect, especially when he ended up sniping his counter-snipers. When he was finally hit by enemy fire—in the face, no less—he still managed to kill the man who'd failed to kill him. Those who met Häyhä later in life tell that he was actually quite a nice and talkative fellow, but undeniably skilled and enough of a terror to his foes to warrant them calling him "The White Death".
Swing Out Sister's lead vocalist Corrine Drewery is known to be soft spoken and generally friendly. However in an interview, she admitted that around "once a year", she can get pissed off enough (her most notable Berserk Button being rude taxi drivers) that she'll "erupt like a volcano", and that those in her close circle actually "recoil in fear" when it happens.
Sergio Gutiérrez Benítez is a short, pudgy, middle aged priest in Mexico who runs an orphanage of over sixty poor children. To earn money for them to go to school, he became Fray Tormenta (Friar Storm), a Luchador. And he kept the secret for over twenty years.
Cardinals fans will applaud for your team if they do something well, but don't rub a loss or a bad call in their faces. Just don't.
African buffalo are usually content to just lumber around and eat grass, but if you try to hurt one, it will attack with the help of its friends and family, regardless of your species. Even if you've got a gun, they'll just hide and ambush you when you least expect it, like a team of gigantic ninja wearing bulletproof shields on their foreheads.
There's nobody in the sport of Mixed Martial Arts who loves his fans or treats them more kindly than Wanderlei Silva. This is the same man who made knocking people out with stomps to the face famous.
Bullying victims who seem meek and docile don't always end up broken. There's a chance that a person who's been bullied and mistreated for a very long time may snap and attack —maybe even kill— his tormentors. Oh, and you don't need physical strength to cause serious harm to people thanks to the existence of weapons.
One such incident was so bad, the battered bully needed a lot of ice packs to simply recover from the pain. It's said that the attacking victim temporarily became superhuman.
The late Roy E Disney, nephew of Walt. Most of the time, a genial old man, champion of traditional animation and of the Disney family legacy. When said legacy was threatened, however...well, just ask Michael Eisner how that worked out.
Xu Chu, a famous warrior from the Three Kingdoms era in Chinese history. Standing nearly 7 feet tall, with a 4+ foot waist circumference, he was known by names such as "Sleeping Tiger", because in battle, he was regarded to be as crazy and as fierce as a tiger, exceedingly gentle otherwise.
Benedict Cumberbatch, otherwise an exceedingly polite, Sophisticated as Hell, charming, caring, adorkable person to anyone he meets, was only bullied once in his life. Because that time when he was threatened? He "pinned [the bully] against the wall, in utter fury, and his assailant stuttered an apology". This is coming from a man who had been nicknamed Bendy-dick Cum-on-my-baps.
As per the Robert D. Kaplan book Balkan Ghosts, the entire country of Romania counts as this. He states that Romanians are very good at passively taking on all manner of abuse, oppression, etc., until they simply cannot take it anymore and unleash a mighty burst of fury. Their soldiers are also very scarily competent, serving the Axis powers well during World War II (to the point of earning the Fuhrer's admiration) until they decided midway through to turn around and fight for the Allieds, which earned them the respect of American commanders for their ferocity.
More recently, in 1989 the people of Romania decided they'd had enough of Nicolae Ceauşescu. The military switched sides after six days, and the secret police were more interested in cleaning out their files than anything else. Three days after the military changed sides, Ceauşescu and his wife were tried and executed. It took twenty-four years to get the people of Romania into the state where they turned on him, and only nine days for them to kill him.