The mothers of the titular Animorphs show this trope in spades towards the end of the series. Some examples:
The Diversion, where the Animorphs finally out themselves to their family, has several examples. Cassie's mother puts her body between her daughter and Ax, whom she thinks is dangerous, mutated wild animal. Rachel's mother grabs a spice rack and faces down what she believes is a full grown male grizzly bear to protect her two younger daughters (unaware said bear is her actually oldest). Tobias compares this to the way his own mother seems to haveabandoned him, but the book ends with her taking a Dracon beam in the back for him too.
Eva, Marco's mother, repeatedly denies opportunities to free herself from the torment of being Visser One's slave (which at one point had her pushed off a cliff and then tortured) in order to help prevent an all out war that would very likely claim the lives of her husband and son. Obviously, badassery is genetic in the Animorphs universe.
Visser One herself is a villainous example. She is a Puppeteer Parasite who conceived and delivered twins through a human host and considers them her own, though she was forced to give them up to maintain her cover. The whole reason she opposes an open war to conquer Earth is because it could kill billions, and those billions might include the only two humans she cares about.
This is Cersei Lannister's main motivation in A Song of Ice and Fire, and her only good trait. Joffrey, Tommen and Myrcella are the world to her, and to protect them from a prophecy that says they'll be crowned and die before she kicks the bucket, she will do anything.
Catelyn Stark also turns into one of these, going so far as to free Jaime "the Kingslayer" Lannister in exchange for her daughters Sansa and Arya, and later she goes on a Knight Templar-like vendetta against the Freys because, among other things, they brutally killed her eldest son Robb.
What about Daenerys "Dany" Targaryen? The woman who caused her to lose her lover Khal Drogo and her unborn baby ended up burned alive to a crisp. She killed 163 people for the agonizing death they gave to innocent slave children. Do not mess with her "children".
Having found the man she believed to have kidnapped her son, Ce'nedra went psycho in his direction. She had to be restrained.
Do NOT threaten ANY of Beldaran's descendants or you will have a very pissed, very powerful Polgara making sure you have a VERY long time to regret it.
In John Varley's story The Bellman, Anna Louise Bach. Think that because she's in labor she can't defend her about-to-be-born child? Guess again. She's got a power drill.
An extreme example is found in Toni Morrison's Beloved, where Sethe, the main character attempts to kill her children in order to protect them from having to go back into slavery. She only succeeds with the titular Beloved.
Beowulf has among the earliest Mama Bears of literature. When the creature Grendel is killed by the hero, his mother attacks and makes her anger known.
In John Gardener's satire-novel Grendel, told from Grendel's point of view, he recalls a time when he was separated from his 'mama' as a child (he actually calls her that sometimes!), with his ankle trapped between two trees, unable to free himself. After a while of screaming for his mama - so loudly the ground rumbles - that a bull comes and attacks him. Why? Because Grendel was going after a newborn calf that he smelled, distracting him from getting home before dawn. After the bull gives up on trying to knock him out of the tree and gore him, Grendel encounters humans for the first time. Who eventually attack him. Just as Hrotgar's ax hits him across the shoulder, Mama comes in with such a fireball of fury, scaring off the humans...and maybe even Grendel himself. And she tears down the trees to make him fall out.
Subverted in The Bible, where Athaliah, queen mother of Ahaziah, responds to the killing of her son the king not by going after the killer...but killing her grandchildren and taking over the throne.
In John C. Wright's Chronicles of Chaos, while Echidna is never exactly safe, it is the death of her son that inspires her to come and slaughter every human being she finds.
A big, quite literal example can be found in Sherrilyn Kenyon's Dark-Hunter series. Particularly in any of the books that mention the Peltiers - A family of Katagari (animals who can turn into humans) bears. You do not mess with Nicolette 'Mama Lo' Peltier or her family. She will beat ten types of crap out of you before you have time to rethink your decision or write out your will.
In the Dear America book A Coal Miner's Bride: The Diary of Anetka Kaminska Americans were throwing rocks at Anetka and her family. She tried to ignore it until one hit her step-daughter at which point she says she became a mad woman like a mother cat.
In the 4th Dexter novel, Dexter by Design, a bound and gagged Rita manages to knock her kidnapper into a running table saw in response to his threats to her children and husband.
This trope sees many uses in Terry Pratchett's Discworld:
Magrat in Carpe Jugulum who disposed of Countess Magpyr in a cold-hearted Mama Bear showdown. While she had a Beware the Nice Ones moment in every book prior to her daughter's birth, it was always a one-off thing triggered by a Berserk Button. In CJ, though, her determination to protect baby Esme makes her possibly the most in-control of the witches (at least in Granny Weatherwax's absence). As Agnes thinks, mothers aren't wet, they're only slightly damp.
Also invoked and deconstructed by Granny Aching in The Wee Free Men, where she teaches a valuable lesson with the aid of a Mama Sheep.
In Mort there's mention of a sacrificial goat that gave birth to twins just before the fatal part of the ceremony, and chased all the priests out of the Temple of Blind Io in defense of her kids.
It's mentioned in one of the books that one of the only creature Greebo ever backed down from was a vixen with pups (this is the Greebo who will chase bears up trees).
Before meeting Sam Vimes, the only thing Sybil Ramkin really cared about was looking after her swamp dragons. In Thud!, upon learning that the dwarf who just got incinerated by said swamp dragons was the fourth member of a team of assassins sent into their home to be killed or captured and the number remaining is unknown, she hands Sam one of the dragons and says "Coal him up."
Glenda in Unseen Academicals, towards pretty much everyone in her sphere. According to Vetinari note who should certainly know — her grandmother was the cook at the Assassins' Guild when he was at school there, and had a standing rule about never using her food to poison anyone, it's a family trait:
'That’s a Sugarbean woman for you, Drumknott, little domestic slaves until they think someone has been wronged and then they go to war like Queen Ynci of Lancre, with chariot wheels spinning and arms and legs all over the place.’
In Divergent by Veronica Roth, the mother of Tris and Caleb.
Stephen King's Dolores Claiborne. She puts up with years of physical and mental abuse from her husband, because her main focus is giving her kids the best life she can by saving for each of them to go to college. But when she finds out that he's hurting the kids, he has to die. She's very methodical about it, arranging to make it look like an accident and ensuring that none of her children are home at the time it happens.
No matter the supposed heroics of the main cast, the prisoners' escape from Pax Tharkas in the first Dragonlance novel (Dragons of Autumn Twilight) would have ended in utter disaster if a certain old, insane, and technically evil dragoness hadn't come through for 'her' (adopted) children in the end and taken on the other (younger and stronger) dragon present all by herself.
The Dresden Files has Charity Carpenter, who is not only a badass with a sword, but fiercely protective of her kids, to the point where she scares even Harry.
There's Susan Rodriguez, who performs a Heroic Sacrifice for her daughter Maggie.
Hints of this with Harry's mother as well. Since Harry is quite a lot like her, this is almost a certainty.
Then there's Queen Mab. Now, she probably didn't give two shits about some two-bit players trying to cause trouble in her court (until they spilled blood), but trying to harm Sarissa didn't exactly earn her favor.
Titania almost kills Harry, despite the fact that he was trying to save the world and everything in it, because Harry killed Aurora in a previous book (for a good reason of course). She had been getting therapy, lucky for him and the world.
Eleven-year-old Candy beats a grown man to death in Emergence, by David Palmer, when he makes the fatal error in judgement of launching a potentially deadly attack with a frying pan on her lifelong pet/sibling Terry (a hyacinth macaw), who is referred to as her "child substitute".
In the same book, a would-be rapist finds out about Mama Bear the hard way when Kim catches him with six-year-old daughter Lisa. Kim's biggest distress over this incident is how long it took to get the mess out of the carpet.
Tash Arranda in Galaxy of Fear is sometimes an odd example. She starts the series at thirteen years old, with a brother a year younger. Feeling Promoted To Parent, she does what she can to help him with their Orphan's Ordeal, but has understandable separation anxiety and abandonment issues. When her brother disappears, Tash believes a Hutt gang lord who threatened both of them before may have abducted him, and goes to beard him in his lair without hesitation. In many other books they're more of a Brother-Sister Team with each helping out the other.
The Hank the Cowdog series brings us Sally May, mother of Little Alfred and Baby Molly. Unfortunately, Hank is quite often the target of her scorn for "corrupting" her children. Still, more than once she's stood up to some pretty serious dangers to keep her kids safe.
Gertie Cat, Though being thin and tired from constant nursing, manages to make a bull run for it's life when it tries to eat the haystack that her kittens are hiding in. She even manages to bring out Hank's Papa Wolf side. When Pete threatens them Hank (quite gladly) pitches him out into a downpour.
The Han Solo Trilogy details Han in his youth and early years as a smuggler, and the first book, Paradise Snare, shows that Chewbacca wasn't the first Wookiee that he developed a close bond with. Serving as a cook aboard the outlaw Garris Shrike's vessel, she came to see the orphan Han Solo as a son, and he reciprocated the feeling. At 16 years of age, Han attempted to strike out on his own, knowing that Shrike would use his abilities as a pilot and scam artist until Han either got caught or was no longer useful. After stealing some supplies, Han met with Dewlanna to say goodbye, but was met by Shrike and his men, and past experience made it clear what was coming. Dewlanna rose to Han's defense and beat up Shrike's men, viciously shattering his brother's arm, before being gunned down by the outlaw. Her sacrifice allowed Han to escape and grow into the man that everyone knows him as.
Leia is also quite the Mama Bear. She does not look kindly on things like kidnapping her children, attempted assassination of her children, or anyone else important to her. Doing any of these things is asking to get your ass kicked at the very least. She gets it from her father.
This is the reason Harry survived his first encounter with Voldemort. His mother Lily was given the chance to flee but she chose to protect baby Harry with her life, which granted Harry magical protection. Notice that this was a young woman in her 20's without her only weapon and in front of one of the most powerful mages ever — and she chose death over handing her baby to the Death Eaters.
Narcissa Malfoy, Draco Malfoy's mother, proves that Even Evil Has Loved Ones: in the seventh book she lies to Voldemort so she can reunite with her son Draco. She may be a pureblood supremacist with the mindset of Screw the Rules, I Have Money!, but she is willing to cross the mightiest dark wizard of all time to ensure her son's safety.
McGonagall has shades of this when it comes to her students, as shown when she defends Harry from Snape. Also, in a castle controlled by Death Eaters, she, despite being noted as old, continues to stand up to people who are known for their hobby of torturing and killing people they don't like, all for the sake of her students. And in the final book when she learns Hogwarts is about to be attacked by Voldemort's forces, she takes charge to protect her students.
In another Diana Wynne Jones novel, Black Maria, Mig's mother becomes very angry when she finds out that Aunt Maria had turned her son into a wolf and tried to get the town to shoot him. Later, when Mig is being held captive in the orphanage (which is essentially used to brainwash the town's children), her mother storms in and demands that the people in charge tell her where her daughter is. All we see is the mother dragging the head of the orphanage and shaking her for locking Mig up, but it's implied that she did more. The children in the orphanage are delighted and cheer her on.
In Death: Areena Mansfield murdered Richard Draco in Witness In Death. Why? Because he was having sex with their daughter Carley. She had told him that she was their daughter because she thought it would turn off his interest in her. Instead, he went and did it, knowing that he was committing incest. He bragged about it to her and wanted to have a threesome composed of him, Areena, and Carley. If you do not consider this a good enough reason for her to go Mama Bear, then you clearly have no soul!
If those cadets come near my Year Sevens again, I will maim them.
Tate, when she finds out her neighbor left her daughter with a child molester.
In Penny from Heaven, Penny relates an incident in which her grandmother threatened Bobby the neighborhood bully with a meat cleaver after he hit Frankie.
In Percy Jackson and the Olympians, Sally Jackson, the loving and nurturing mother of Percy, was shown fighting in the last battle using a stolen shotgun from the police.
Yanaba Maddock, the protagonist of the Petaybee books, becomes this in the spinoff series The Twins of Petaybee.
Sidney Sheldon's Rage of Angels reveals lawyer Jennifer Parker to be a version of this, though she has to work indirectly. When her son is kidnapped, she knows time is of the essence, so she [calls on the Mafia prince who's been lusting after her — whom she knows to be very bad news from personal experience — and asks him to do whatever he can to bring the child back alive, and when asked what to do with the kidnapper, she says "Kill him!" He mobilizes his forces to track them down, the child is rescued, and he kills the man himself. The two adults become lovers after this, setting up the remainder of the book.
Coupled with an instance of Good Stepmother in Ragnar Lodbrok and His Sons: When her stepsons Erik and Agnar are killed by Eystein Beli, Aslaug drives on her biological sons to avenge them, and personally leads an army to battle Eystein, who is eventually defeated and killed.
In The Red Tent, when she finds out that her husband Laban had been fondling Leah and Zilpah, Adah proceeded to beat the ever-living crap out of him. When he was just inches away from dying from the resultant injuries, Adah swore that if he did it again, she would not only beat him up again, but that she would call upon the Powers That Be and have them give him some Laser-Guided Karma in the form of disease and impotence. Laban then made sacrifices to the gods, had a statue of one of their chief goddesses made, and bought Adah and his daughters jewelry...and never touched the girls again.
She also brought it up again, when he had made a deal with Jacob to marry Rachel off to him, because Rachel had not yet menstruated. In all fairness, Laban didn't know that, but the threat was enough to get him to comply with his wife's wishes, and extend the time Jacob worked for him.
Inverted in Skulduggery Pleasant: Death Bringer when Valkyrie hears her mother was attacked by a mugger and breaks into his cell to beat him up.
In The Secret of Platform 13,Larina Trottle is a villainous example, using her vast resources to move heaven and earth to protect her son from kidnappers. (Despite having kidnapped a child herself to get a son; these new kidnappers, in fact, are trying to restore him to his rightful family.) She also plots to destroy the life of her servant boy, Ben, for the simple crime of being smarter and more generally likeable than her little darling. (For added hypocrisy, Ben was the kid she kidnapped and planned to raise as her son, until she got pregnant and tossed him aside.)
Although Sarah Heap usually proves that Adults Are Useless, she can become very vocal if her family or any of her children are threatened in any way, as Marcia Overstrand had to find out the hard way in Flyte:
Queen Cerys's, Jenna's mother, job is to keep her daughter safe from harm, as she does by throwing Queen Etheldredda out of the Queen's Room in Physik.
In Michael Flynn's Spiral Arm novel On the Razor's Edge, Bridget calls in the Hounds to get aid: they are going after her daughter into enemy territory. Later, one Shadow is surprise at how many Hounds followed; another is not, because she had stolen the cub from the Mama Bear.
A rarely discussed, but still notable, example would be Mara Jade Skywalker, from the Star WarsExpanded Universe. Probably most notable in Sacrifice, where her worry for her son (Ben) convinces her to go hunt and attempt to kill the two Sith Lords whom she thinks are plotting to kill him. She's partially right in that respect, and this leads to a VERY nasty brawl.
Meg in Stork Raving Mad. Think that because she's in labor with twins she can't defend her about-to-be-born children? Guess again. She's got a hat rack.
John Wyndham's short story Survival, in which one of the stranded passengers resorts to extreme measures to make sure her baby doesn't starve.
Bliss, Susan's hippie mother, takes someone out with a statue of Buddha to protect her full grown daughter in Sweetheart.
Rachel O'Neal, in Teetoncey by Theodore Taylor, is this to a young shipwreck victim whom she nicknames "Teetoncey". Do not start with Rachel O'Neal. You will not win. That includes you, Atlantic Ocean.
In The Tenant of Wildfell Hall, Helen's fears for her son and not her own misery force her to take him and run away from her husband. While this may seem like logical behavior now, at the time the novel was written this was illegal, scandalous, and unheard of.
Time Scout: Ianira. In Wagers of Sin, the fact that Skeeter was trying to rescue the father of her children gave her words extra weight, for both uptimers and downtimers.
Lady Svetlana in Tranquilium is very much this after giving birth in the beginning of Part Two. She goes out of her way to protect her little boy from revolutionary terror, war and a scarily ruthless conspiracy based in another dimension, though she got help along the way; for all her otherissuesandwhatnot, the safety of her child clearly becomes an overriding concern for her for the rest of the book. This status of hers is also helped somewhat by the fact that she is actually pretty good at fist-fighting thanks to the training she underwent with her father's men (her father was a captain).
In Unfinished Tales, Christopher Tolkien describes a Worthy Opponent version of this trope. In the wars between Gondor and the Wainriders, a revolt is arranged among some of the Wainriders slaves while the men are away on campaign:
"But most of them perished in the attempt; for they were ill-armed, and the enemy had not left their homes undefended: their youths and old men were aided by the younger women, who in that people were also trained in arms and fought fiercely in defence of their homes and their children."
Also, in the Book Of Lost Tales version of the Fall of Gondolin, Idril is described as fighting 'like a tigress' to protect her son Eärendil from Maeglin. She fails, but when Papa Wolf Tuor turns up...
Princess Kareen in the same series makes a very credible attempt to kill her unwanted lover when she realizes the man is a threat to the life of her five-year-old son, Gregor. She fails, but not for lack of trying.
Many queens in Warrior Cats behave like this whenever their kits are in danger or bullied. Several examples include Yellowfang, Leafpool, Sasha, and Sandstorm.
Averted with Rainflower, the mother of Crookedstar and Oakheart, who cruelly neglects Crookedstar all because he broke his jaw. Thus, it's Shellheart who brings out his Papa Wolf roll.
World War Z has a woman who goes into a blind rage when a zombie tries to get her daughter. Her children told her later that she had ripped its head off with her bare hands. There was also the nun who held off a zombie horde for quite some time with a large iron candlestick, protecting the children from the local Catholic school.
Nita Callahan's mother in The Wizard's Dilemma turns into one of these when faced with the prospect of the Devil-equivalent taking Nita's soul. Her line is "She is still my daughter, and she does not have my permission!"
In High Wizardry, Dairine Callahan becomes the "mother" to a race of sentient silicon lifeforms, and a threat to them (as well as Nita and Kit) prompts her to tell the Devil-equivalent, "Touch them and you're dead meat." Shortly after, when a powerful ally appears to help out, the ally's gender turns out to be female, surprising the protagonists, said ally then comments "Men will fight bravely and be heroes, but for last-ditch defense against any odds . . .get a Mother."
Averted in Frostflower and Thorn because while Thorn mounts a daring rescue for her biological son Starwind she's doing it for Frost's sake, not his, and while Frostflower would give her life for her son she would not take a life for him, especially since she knows he won't be harmed by the priests.
Mere Lessard of Sacré Bleu nearly brains the titular muse with a crepe pan when she believes that Bleu is endangering her son. She earlier gives a few gruesome examples of what she will do to "Juliette" if Lucien comes to any harm whatsoever.
Jodi Picoult's novel Perfect Match is about a woman who shoots and kills a man who she believes molested her five-year-old son. Tragically, he turns out to have been innocent.
In M.T. Anderson's novel He Laughed with His Other Mouths, Dolores Dash battles an evil alien who has her son under mind-control. She causes a building to collapse on the alien.