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. Large family 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.
Steve Irwin. One of the nicest guys you could ever hope to have met, but you did not ever brag about abusing animals in his hearing distance, ever.
Members of the Religious Society of Friends (more often known as Quakers) are defined (in part) by their testimony against war. You'd imagine they'd be the nicest, kindest, most docile and timid opponents you'd meet. But some Quakers have been pushed too far, and have taken up the sword, sometimes with spectacular results.
Gen. Nathaniel Greene was, by the end of the Revolutionary War, second only to George Washington on the roster of American Generals. Leading troops in almost every major victory and defeat under Washington in the northern theater, Greene was sent to command the Southern army towards the end of the war. There he "lost" three battles in a row, all Pyrrhic victories for the Army. The Brits had to give up Georgia, South Carolina, and North Carolina to the rebels. A Quaker ended up forcing the Crown to evacuate the southern half of his Majesty's colonies. Beware the Nice Ones, yes indeed.
Edwin Stanton was a Quaker, and Abraham Lincoln's Secretary of War. A capable administrator, unfailingly honest, and one scary man if you were to run afoul of him. Spies and subversives were treated harshly.
Gen. Isaac Peace Rodman joined the Union Army at the start of the Civil War, a lowly captain. One year and three months later, he died commanding a division of soldiers (a very speedy advancement of four ranks due to his competence and bravery), at the Battle of Antietam.
Gen. Smedley Butler won two Medals of Honor. Fought in China. Fought in Mexico. Fought in Central America. Fought in Haiti. Fought in WWI. Fought mobsters in Philadelphia as Director of Public Safety. Then he wrote a book in the 30's about how he hated fighting, called "War Is A Racket" (essentially picking a public relations fight with Wall Street, Big Oil, and Big Banks). There's no doubt he hated fighting... but neither is there any doubt that he was really good at it, no matter who the opponent.
All of the above came about when individual Quakers got pushed to violence. The group as a whole, while remaining nonviolent, also has a pretty impressive track record when pushed too far: helped end slavery (John Woolman), taught Martin Luther King how to win a fight without shooting (Bayard Rustin), and got women the right to vote (Lucretia Mott and Susan B. Anthony). Basically if you make them angry, you'll probably survive... but you'll probably still lose eventually.
Consider these when next opposed with a Quaker. They're not always the happy, kindly gent on the oatmeal drum.
There's an old Quaker saying, "I would not harm thee for all the world, friend, but thou art standing where I am about to shoot."
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.
The Swiss. Yes, the Swiss, of chocolate-and-clock fame. Swiss mercenaries were known for their consistent record of victory and refusal to take prisoners. The kings of France thought it impossible to go to war without Swiss pikemen as the core of their infantry. Eventually, using Swiss mercenaries was declared a war crime and today the only Swiss troops you tend to hear about are the ones who make up the Vatican's Pontifical Swiss Guard. Yeah, these are the guys who have been trusted to guard the Pope for five hundred years and counting. Do not screw with.
The reason they're neutral is that for hundreds of years no one has been stupid enough to start a fight with them.
There's story which takes place back when rifles could fire five shots. It goes like this:
A German commander and Swiss commander are chatting. The German says, "In 24 hours, I could summon up 25 million troops." The Swiss nods and says, "In that time, I could call up 5 million." The German laughs. "And then what?" The Swiss simply replies, "And then my men would line up, empty their rifles, and go home for dinner."
A variation: The Swiss commander says "I could call up an army of 1 million." The German commander asks "What if we send an army of 2 million?" The response— "Then we'll shoot twice."
They also froze out several enemies, much like the Russians.
Don't ever fire a gun at anyone in Switzerland. Even now, it's perfectly legal for any bystander with a gun handy to shoot you back. And probably with better aim than you have.
If you ever go hiking in Switzerland, you'll see a lot of roadsigns with bullet holes in them. Yep, they do practice a lot. Oh, and they take their service rifles home with them when their time in the army is over.
Most citizens you meet who aren't in active service are in the reserves. Swiss reservists are required by law to have their military rifle and a box of ammunition in their home in case they get called up. (Yes, the box is sealed, but that's to ensure they have the required quantity of ammunition. It's not difficult or expensive to acquire more to practice with.)
Also, every single bridge and mountain pass is wired with explosives, which can be detonated in case of invasion to block off invasion routes and allow defensive formations. And that's before you get into the hidden jet fighter hangars in the mountains; a door opens and a Tiger rolls onto the road and takes off.
Wisconsin. A very pleasant state in the American Union, it's home to a lot of amiable and friendly people. It also boasts a disproportionately high number of the union's most infamous serial killers.
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".
Australians are generally considered to be some of the friendliest, most charming people on the planet. Yet the Australian military has a history of laying brutal beatdowns on their adversaries in wartime. During the Vietnam War, the North Vietnamese were much more afraid of fighting Australians than Americans. The reason being, when an American unit made contact, they'd disengage and call in air support, giving the NVA time to get out of the way and disappear. When the Australians made contact, they just tried to get even closer.
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.
St. Louis Cardinals fans are generally accepted as the nicest of all the Major League Baseball fanbases. But if you're President Obama in a White Sox jersey, they will boo you. If you're George W. Bush, they will boo you beneath the cheers piped in over the speakers. And if you're Albert Pujols and you just signed a $250 million contract with the Los Angeles Angels of Anaheim, they will burn their Pujols jerseys. Until that one nice pastor suggested that they donate the jerseys to his church to send to homeless shelters in Anaheim.
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.
Bronies can be ridiculously friendly to complete strangers, showering them in bro-hooves. But make fun of My Little Pony: Friendship Is Magic around the less stable ones and you can end up on the floor unconscious for two thirds of the show. Fortunately, most of them would either say, ‘Different strokes,’*
Note that some of the Yanks With Tanks are Bronies, and they can be found even in the special operations divisions.
The Irish are almost always this. The page quote at the top is actually derived from an Irish saying "Beware the wrath of the quiet man." You may not think it by looking at Ireland's history of being a subject nation of the British Empire, but Irish mercenaries were used to a ridiculous extent in the wars of western Europe for most of history post dark ages and kicked ludicrous amounts of ass as auxiliaries in many armies. Some modern armies still have Irish Regiments, Britain has at least 3.
Of note is the troubles, without getting into the hows and whys, at the end of the day when Irishmen feel something they hold dear is threatened, it results in a bad day for all involved.
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.
The Gurkhas of Nepal are generally a group of fairly nice, calm peaceful people. However, there is a reason why they make up some of the most fearsome regiments in the British Army.
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.
The Italian people as whole is rather nice and laid back, but when they get in a war they believe into... Well, at the end of World War I the commander in chief of the Italian army was rightly bragging to the world to have just annihilated one of the strongest armies in the world (in fact the Austro-Hungarian Empire was the only member of the Central Powers to see its army crumble and surrender inconditionally before while the triumphant Italian army was still busy returning to the pre-war front), and in World War II the German forces occupying Italy from 1943 tended to sent Fascist collaborators to fight the Italian partisans due the extreme violence of the fights against the partisans, violence only matched by the Fascist soldiers.
Once in a while, even Italian soldiers fighting a war they don't fully believe in can perform this trope, as the US Army learned the hard way at Kasserine Pass (they managed to hold back Rommel's German troops, but crumbled when he sent the Bersaglieri assault troops).