Avatar: The Last Airbender gives us Princess Ursa of the Fire Nation, who used to provide the main page quote: "That's what moms are like — if you mess with their babies, they'll bite you back." ; she knows what she's talking about, having killed Fire Lord Azulon for ordering her son Zuko's death. There is somedebate as to whether this Mama Bear attitude applied to Azula as well; we didn't see enough of Ursa to really know what was up with her.
It's likely that Ursa loved her as well and was concerned for her, seeing as she was probably the most normal and compassionate out of that entire family, but Azula's deteriorating sanity and yearning for approval warped her perception of her mother.
Additionally, Ursa appeared to be disturbed by Azula in many ways, the times Azula spoke so flippantly about family members dying being prime examples. She was also the only one who attempted to discipline Azula; you can bet neither Ozai nor Azulon would've curbed her growing sociopathy, as those traits are valued in a militaristic nation. Even a parent would find a child with those characteristics hard to love. So whether or not that Mama Bear instinct would kick in if her daughter was threatened is left solely to speculation.
Also, Katara's mother Kya, who pulled a Heroic Sacrifice to keep the Fire Nation from finding out her daughter was a Waterbender.
Not only did she lie about being a Waterbender, she did so right after telling her daughter that everything was going to be okay. With the Southern Raider's commander standing right in front of her, that took a great deal of courage. This is why you don't screw with the Water Tribe.
Avatar Kyoshi may have been motivated by this trope when she split her home town off the mainland into its own island, as her backstory reveals she bore at least one child.
In a milder, funny example, the saber-tooth moose lion.
The turtle-duck that attacked young Zuko, prompting Ursa to state the top-quote.
Kyle's mom Sheila in South Park takes this trope a bit too far, as she starts World War III and nearly triggers Armageddon all in the name of protecting her son from toilet humor in the media.
Danny Phantom where at least two episodes have the main character's mother, Maddie Fenton, kicking all sorts of ass towards a group of ghosts to protect her son. One of which involves an Ecto-induced lightsaber. She's a 6th degree black belt who was already an Action Mom and whose response to a threat not directly aimed at her son is normally overkill. When she goes Mama Bear, prepare for a butt kicking.
She also gets a moment with the Straw Feminist Gloria Ironbox. At first, she just makes some snarky comments about Gloria's extremist opinions, but then the offending strawwoman insults her children. Cue the Cat Fight.
And you do NOT want to steal her baby's candy. For that matter you don't even want your KIDS taking her baby's candy. Seriously, she's harsh.
In "Meg and Quagmire", after she and Peter save Meg, who just turned 18 and is now legal, from having sex with Quagmire, Lois threatens to CUT OFF HIS PENIS AND FEED IT TO BRIAN if he ever touches Meg again.
At the end of the 'Odyssey' arc of Gargoyles, we find out that Fox is actually the half-fey daughter of Titania, the Queen of the Fae, but she's been unable to use her magical powers due to being raised amongst humans. Then Titania and Oberon try to steal away her newly born baby, Alexander, and a united front by the Gargoyles, Cyberbiotics, Xanatos and (in a last-minute effort) Puck, fails to stop them. Then, Fox proceeds to blast Oberon through a wall in her first and last display of her true powers. It may actually have been ''planned'' that way by Titania, in an effort to force Fox into displaying her potential by deliberately invoking this trope...Xanatos isn't the only one in the series who can throw a good gambit.
Also, Demona in the episode 'The Reckoning'. She and Thailog have the Clan at their mercy - until Thailog threatens to kill her daughter Angela, at which point Demona first turns on Thailog and then frees Goliath and the rest of the Clan rather than allow her daughter to be killed.
Teen Titans: In the episode "Hide and Seek," Raven has to escort three very young superpowered kids to a monastery. She resents the job at first, but eventually comes to care for her charges, culminating in the line "Nobody messes with MY kids!"
Justice League Unlimited has a small example with a nameless woman. A mother and her daughter are trapped in a car that just fell from a suspension bridge. Hawkgirl comes in and rips the roof off with her mace. Rather than wait for rescue, the mother does not hesitate to grab her kid and throw her up to Hawkgirl. Hawkgirl rescues both of them. Even the nameless extras in the DCAU are badass.
Peggy Hill from King of the Hill has a tendency to do this. When her niece/adoptive daughter Luanne's biological mother, Leanne, returns and starts treating Luanne like crap, being a crappy mom in general, and as the final straw, humiliating her by giving Luanne's boyfriend a lap dance in front of the whole neighborhood, Peggy finally snaps and tears into Leanne in a No-Holds-Barred Beatdown.
Also inverted: when Bobby kicks Hank in the groin and then takes advantage of his injury to calmly play a video game, even though he was banned from doing so, while forcing his father to limp around the house after him. When she sees this, Peggy attacks her own son, even giving him a noogie, because "If you show disrespect toward the man I love, you are going to have to deal with Peggy Hill!" Hank then reinforces her demonstration by telling Bobby "I can get her to do that any time, you know."
The Secret Saturdays: Drew Saturday. Mess with her "boys" (her biological son and a cat-gorilla cryptid) and prepare for a world of hurt. She's already an Action Mom trained in martial arts, armed with a Flaming Sword, and has survived conditions and situations on a daily basis that would kill an average person, and is part of a Bad Ass Family. And that's when she's not in Mama Bear turf.
On The Simpsons, Marge gets her own moments every now and then.
When she was treating Nelson all nice, and giving him money. Nelson's own mother came by and yelled at Marge. Her reply? "Go home, Mrs. Muntz...AND TRY NOT TO HAVE INTERCOURSE ON THE WAY!" The reaction of her family was priceless.
In "The Great Wife Hope", Marge ends up fighting the guy in charge of a shallow pastiche of Ultimate Fighting. She gets one-punched to the floor, Bart comes to her aid, and when her opponent picks up Bart and threatens to hit him, she growls, "That's...my...SON!" and proceeds to beat up the guy. Do NOT F*** with Marge.
Both of Timmy's parents, despite being comically irresponsible enough to leave him with an evil babysitter, have gone to crazy lengths to protect Timmy. This came to the fore when Timmy wishes they had superpowers, and he ends up using this to his advantage when convincing them to give up their powers to save him.
Helen Morgendorffer in Daria. Never has a matter of fact threat of litigation been so Badass:
Helen: "Let Me Get This Straight; you took my daughter's picture, altered its content, displayed it against her will and now you're threatening disciplinary action for vandalizing a picture that you admit to stealing? ...Ms. Li, have you ever heard of violation of civil liberties? ...and big fat lawsuit?!"
Hunky, from Max and Dave Fleischer's Hunky and Spunky cartoons, was a Mama Burro. When the other barnyard animals ganged up on her bratty offspring, Spunky, she whooped them all to save her baby (then proceeded to spank Spunky for still being a brat). And God help any prospectors who think they can turn Spunky into a pack mule...
Eve from Alpha And Omega. Put one paw on her daughter and you're in for a world of pain. This quote says it all:
Somewhat off-screen in Winx Club and crossing with Papa Wolf is Miriam (Bloom's Mom). After both she and her husband think both their daughters to be dead, they according to one account went completly berserk at the ones responsible; the Ancient Coven. Considering how high they are in among other's Faragonda's and Baltor's regard, that says something about the power which was unleashed. Ouch...
In Codename: Kids Next Door, Numbuh Three is like this towards Bradley, a skunk that she finds in "Operation: C.A.M.P." and assumes to be an orphan, showing maternal instincts towards him and being fiercely protective towards him. (Bradley's actual mother - and his father - who show up at the end of the episode, are the same.)
In the original My Little Pony episode "Bright Lights," the mothers among the ponies were more than ready to tear Knight Shade limb from limb after the baby ponies' shadows were stolen, which meant that they'd soon become Empty Shells.
And then there's Knight Shade's own mother, who's willing to stand between her son and an angry mob of ponies who had their shadows stolen. She does give him a stern talking to, however.
Cybersix is this to Lucas, but also very much to Julian. In Julian's introductory episode, we see that this is because he reminds Cybersix of her dead brother who she failed to save when he fell off of a cliff right next to her.
Rebecca Cunningham of TaleSpin is a literal example, while usually not much for the teams' action packed heroics, she really steps up a mile whenever Molly is threatened. The others tend to suffer quite an earful whenever they inadvertantly put her in danger as well.
Brave, Elinor is a Mama Bear in the film's climax. Literally.
Princess Bubblegum displays a very interesting variation of this in Adventure Time. During the episode "You Made Me!", she realizes that her failed science experiment Lemongrab is not only desperately lonely, but completely insane. She spends the entire episode trying to help him sort out his issues, even as he yells at her, threatens her, and acts like a jackass. It pays off in the end, with one of the most heartwarming moments in the series. Initially, Princess Bubblegum treated Lemongrab like a nuisance and an experiment, (and did so in the earlier episode, "Too Young.") After Lemongrab made his Despair Speech, she began to sympathize with him, and treated him like a person. And at the end, once he's well and happy with his friend (clone) Lemongrab, Princess Bubblegum bids them both good-bye with the words, "Good-bye! Take care, boys!"
Arcee in Transformers Prime is normally a Cool Big Sis to Jack, but will lay down a serious amount of hurt on anyone who threatens his safety. This extends to verbal lashings to teammates who inadvertently put him in harm's way.
Francine Smith of American Dad! can be one, particularly in "Bully for Steve", which reveals she doesn't take her husband's bullying of her son very well.
Hayley becomes this in "Season's Beatings" to her newly-adopted son, who just so happens to be the Anti-Christ.
Both Ursa Barbic and Grammi Gummi from Disney's Adventures of the Gummi Bears can serve as literal Mama Bears. Grammi takes this trope in an every day occurrence, defending Sunni, Cubbi and Tummi from Gruffi's rather harsh and uncaring attitude. When the situation gets rough, she is not afraid to venture into enemy territory to protect her charges, this protectiveness extends to Princess Calla and Cavin as well when they come into their lives. Ursa Barbic, of the Barbic Gummies, takes a more distant approach: watching from afar and trusting Buddi to be able to handle himself. However, when the danger grows, such as when Buddi was captured in Season 5's "return to Ursalia" the only thing stopping her from using their own Ultimate Weapon to blast Buddi's captor away was the knowledge she would hurt him as well.
Wander Over Yonder: Although normally a Deadpan Snarker who is often annoyed by his energy and love for adventure, Sylvia is very protective of Wander, as confirmed by Craig McCracken. She even resembles Ripley in an episode where she thinks he's dead. She also becomes quite protective (although reluctant to admit it) of an egg Wander orders to look after.