Arguably, Chris Parker in Adventures in Babysitting. OK, she's the babysitter, not the mom, and two of the kids aren't that much younger than Chris herself. Still, a suburban teen takes on several inner-city gang members to defend her charges while in the way to help her best friend. Successfully. With verbal intimidation alone.
Chris: (grabs the Lords of Hell gang leaders knife and points it at him) Don't Fuck with the babysitter!
Pictured on the main page: Ellen Ripley of the Alien series was always a competent character with a decent survival instinct.Threaten her quasi-adoptee girl Newt and then, well, she's duct-taping weapons together to make a BFG, single-handedly invading the aliens' nest and strapping into a Power Loader to take on the Alien Queen in the mother of all catfights. Say it with us: "Get away from her, you bitch!" And considering that Ripley had killed a number of the Queen's offspring as well, it qualifies as a Mama Bear Showdown.
Animal Kingdom contains an evil example in the shape of Janine, grandmotherly matriarch of the criminal Cody family and fiercely protective of her sons to the extent that she arranges for her grandson Joshua to be killed after he threatens to testify against them.
If a character played by Jodie Foster has a child or a teenager in her care — do not threaten them! Movies like Flight Plan (where her little daughter is abducted and petty much erased by a bunch of terrorists playing a Gambit Roulette) and Panic Room (where she and her teenage daughter, also an Ill Girl, have to protect themselves thee dangerous delinquents searching for something in their apartment's panic room]]) should make that very, very clear.
Do NOT steal Mothra's egg and/or try to harm her larvae. She will unleash her rage upon you....Yes, even if you happen to be GODZILLA. Mothra lost the fight. But she bought enough time for her larvae to hatch. Said larvae then proceed to hand Godzilla one of his few defeats ever by wrapping him up in silk. Yes, freaking Godzilla got his ass kicked by infants.
In Godzilla (2014), when the Female Muto realizes her nest has been blown up she immediately ditches the fight with Godzilla, and when she sees Ford nearby she puts two and two together and relentlessly chases him.
The British Kaiju film, Gorgo features a 200-foot tall monster that's nearly indestructible to all of man's weapons such as the military, bombs, fire, and electrocution, and is on a Roaring Rampageof Revenge all over Great Britain. Why? Because her child has been captured and is on display as a sideshow attraction at a circus for people to laugh at.
Homeward Bound: The Incredible Journey had a very literal use of the trope with Chance trying to "fish" with Sassy's help. The fish ended up getting close to some bear cubs. Chance, instead of giving up his fish, decided to scare away the bear cubs. The bear cubs then turned tail and fled from Chance's barking, and climbed up a tree screaming for their mother. Sure enough, while Chance was bragging about his victory, their mother approached Chance, not too happy with him for what he did and fully ready to kill him for it. Chance was just barely warned before he just barely fled.
Amusingly, the cubs were black bears while the "mother" was a grizzly bear.
Getting her entire wedding party ambushed and blown away by Bill and his assassins was bad enough. Getting the ever-loving crap beaten out of her by the assassins, followed by a bullet in the head from Bill himself, was bad enough. But when the Bride finally gets out of her coma after four years and discovers that the child that she was carrying is gone (and apparently dead), she decides to...well...Kill Bill.
Also, The Bride may be a Dark Action Girl, but she does not like it when children (like Nikki and the ultimately alive BB) or teenagers (like Gogo or the young yakuza she spares) are caught in violent affairs. And she won't hesitate to say it.
The premise of The Last House on the Left is about a group of people who get trapped inside a house with a Papa Wolf and a Mama Bear after they brutalize and rape the couple's daughter. Three guesses as to what happens next.
This was the point of The Long Kiss Goodnight, where Geena Davis plays an ordinary, suburban homemaker/schoolteacher...until an escaped convict threatens her husband and child. Then it suddenly turns out that she's a former assassin with Easy Amnesia, and she proceeds to kick his ass seven ways to Sunday. Towards the end of the movie, she's taking on a terrorist-organization-backed-by-CIA single-handedly to save her daughter.
The Lost World: Jurassic Park combines this with Papa Wolf, showing that the mated T-Rex couple are fiercely protective of their offspring; the human protagonists quickly note in a frightened tone that this completely debunks a widely-held theory that the species would abandon their young quickly after they were hatched.
In the third film, the alpha velociraptor ordered her pack to hunt down the protagonists after one of them stole her eggs.
Non-action-movie version: In the 1938 Yiddish film Mamele (Little Mother), Khavkhe (Molly Picon) takes care of her whole family. A gangster, Max Katz, recruits her brother into a burglary gang, and courts her sister (with the intention of turning her into a prostitute). Khavkhe manages to pickpocket Katz's gun, and makes him write a letter breaking up with the sister. ("You may be a Katz (cat), but you'll leave here as a kitten.")
Evelyn in The Mummy Returns: Don't kidnap her son, or she'll even come back from the dead to kick your ass.
Mercedes from Pan's Labyrinth, considering the fact Ofelia's Ill Girl mother is too weakened from her very difficult pregnancy to really be there for her daughter. Mercedes is really the only one throughout the movie who is consistently kind to Ofelia and goes Mama Bear on Vidal:
And even better when Captain Vidal is mortally injured after having done the same to poor Ophelia, and Mercedes tells him that she'll make sure newborn child will never have memories of his evil and now dead father.
Raising Arizona: Even though she and her husband stole their "son", Edwina "Ed" McDunnough gets mighty protective of him when the evil bounty hunter comes for him.
"Give me that baby, you warthog from Hell!"
In Salt, the reason why Evelyn Salt abandoned her mission and faked the Russian Prime Minister's death was so she could save her husband. But once Orlov had her husband killed, she decides to go on a Roaring Rampage of Revenge to hunt down every other Russian agent who was involved.
Jasmine, the aging lioness in Secondhand Lions, might be getting old and a little bit sickly. She might not be too much to look at any more. But put one hand on her human "cub" Walter, and you are in for a world of pain, even if the strain is too much for her heart.
Rose De Silva is willing to go to hell and back to get her mentally troubled daughter Sharon back in the film version of Silent Hill. Along the way, Rose finds that Sharon didn't have mental problems, she was all that was left of a 30 year old woman's innocence, and she was being called back to Silent Hill.
Surf Nazis Must Die has a mother avenging the death of her son against the eponymous Surf Nazis.
The Terminator: Sarah Connor. Saving the world from nuclear annihilation is considered an incidental bonus to saving her son from the machine sent back to kill him. This gets cranked up to eleven in The Series.