She is wedded to convictions—in default of grosser ties; Her contentions are her children, Heaven help him who denies!— He will meet no suave discussion, but the instant, white-hot, wild, Wakened female of the species warring as for spouse and child.
A literalMama Bear appears in mothy's "Moonlit Bear", in which the protagonist, Eve (portrayed by Miku), steals two "apples" from a bear, which then proceeds to chase her through the forest. Turns out that the Mama Bear is a human mother after all, and the two "apples" are actually her twin children, who she is desperate to retrieve. Out of fear as well as her desire to have children of her own since hers are deceased, Eve ends up murdering the "bear".
Persephone's mother Demeter, goddess of the harvest, refuses to allow any growth of crops until her daughter is returned to her from the Underworld, resulting in many mortal deaths. Made more interesting in that Persephone's kidnapper/husband is also her uncle, Demeter's younger brother Hades.
Well, part of her motivation for hating Agamemnon comes from his treatment of her children by her first husband—in order to marry her, he invaded Mycenae, killed her husband, and dashed her baby's brains out against a wall. In front of her. With her father's blessing. So when he killed another one of her children, this time also one of his own, in pursuit of her perfect sister Helen, whose daughter was promised to Orestes by mutual agreement of the fathers, it's not as surprising that she got a little crazy.
Pele, the Hawaiian goddess of fire. According to legend, she regards all the volcanic rocks on the shores of the Big Island to be her children, and will inflict a horrible curse on any tourist who dares to take one of the rocks home with them as a souvenir
Rose Is Rose played this trope completely straight with a Sunday strip, featuring Rose transformed into a "Mama Bear" to symbolize her overprotective nature before she meets Pasquale's new babysitter. However, Rose relaxes when she opens the door she finds that the prospective sitter is apparently a Mama Bear as well.
"Tragedy struck when Conroy, his mind preoccupied with work, stepped into the elevator—directly between a female grizzly and her cub."
Dino Attack RPG has Sarah Bishop. She even got a moment homaging the climax of Aliens. At the same time, however, Sarah Bishop is partially a deconstruction. She (understandably) concerned about her daughter being in a war zone, but her own Adult Fear made her completely psychotic to the point that many players expressed genuine fear of her. This same thing also broke her psychologically once the war was finally over and she'd realized what she had done.
Unknown Armies has the Mother archetype. High-level Avatars of the Mother get huge bonuses in combat when they're either defending a child, or pregnant. The phrase "mama bear" even shows up in the rulebook description of their second Avatar channel.
Munchkin invokes this with the "Mommy" card. When someone is in combat with a weaker monster or one that's been turned into a Baby, you can play this card to summon the monster's mother, who is ten levels higher. And woe is you if the Baby+ Mommy combo is applied on a Lv. 20 Plutonium Dragon...
1st Edition Monster Manual. When a pair of mated dragons had offspring, any attack on the young dragons caused the father and mother to attack the intruders with a ferocity bonus of +2 to hit and +1/+3 on clawing/biting damage.
1st Edition Monster Manual II. If a female cheetah's cubs are threatened she will attack at a +2 to hit and do +2 Hit Points damage per attack.
Mayfair Games' 1E Role Aids supplement Lizardmen. When female lizardmen are guarding the eggs in the hatchling pool, they fight at as level 3 fighters (instead of their normal level 1), a bonus given them by the lizardman deity Rega.
The Grey Render◊ is a huge monstrosity with a penchant for "adopting" small creatures of other species, to whom it will thereafter bring food and protection. And if you dare so much as look at them funny, it will tear you down like the squishy little piece of meat you are and share your body with its protegés.
In GURPS Bestiary, a sidebar makes it very clear why you shouldn't get between a mother animal and her young.
Mothers protecting their young are the fiercest foes on Earth, and probably anywhere else for that matter. Mother animals know no fear, are quick to feel threatened, take offense easily, and will attack viciously, giving no quarter.
Marvel Super Heroes Adventure Game (SAGA System) Adventure 3 Fantastic Four: Fantastic Voyages. In the adventure "Wild at Heart", a monster emerges from the earth in a small town and starts destroying the place. If the heroes don't subdue the creature quickly enough, a much larger monster appears and starts searching for the smaller one - it's the smaller monster's mother. If the heroes have hurt its child it will go berserk, trying to destroy anyone and anything it can see.
Chaosium's supplement All the World's Monsters. Female bears and tigers (if cubs are attacked, fight at +2 to hit) and Elder Daughters of Kali (do double damage to anyone attacking her children).
The Morrow Project. Female bears and pigs are noted as being especially protective of their young and prone to attack if they are threatened.
Champions Organization Book The Circle and M.E.T.E., adventure "The Hatching". The M.E.T.E. (Metropolitan Extra Terrestrial Enclave) receives a large spherical object (called Case 39) that was found in a national park. It eventually hatches out an alien Blob-like monster which goes on a rampage. Three months later the alien's mother shows up and tries to rescue its child, and heaven help any superhero that gets in its way.
Star Fleet Battles. The monsters known as Space Dragons sometimes ravage inhabited worlds. If a ship gets between a Space Dragon mother and its child, the mother will change course, move toward the ship and attack it.
Kim from Miss Saigonshoots her own cousin who tries to kill her illegitimate son by an American GI, yelling "I have no other choice, what I must do I will!" as she does so. A lot of people remember the part as a defining moment for Kim, especially with her screaming "YOU WILL NOT TAKE MY CHILD!"
The interpretation of Umbridge in A Very Potter Sequel likes to consider herself a mama bear In-Universe — ""It's your Mama Umbridge's job to keep her baby bears safe!" However, she doesn't actually fulfill this role and just embarrasses the girls.
Morgan Fey from Ace Attorney will do anything to take care of her daughter Pearl. More specifically, to make sure Pearl becomes the Kurain master rather than being shunted to the side while Maya takes the position. Not only did she aid a murder and frame Maya for it, she then tried to make Pearl channel the spirit of Dahlia (Pearl had no idea that Dahlia was her dead older half-sister) so Dahlia could murder Maya. We didn't say she was nice.
In answer, and in a much more heroic example, Morgan's younger sister Misty Fey totes around a sword-cane and swears to do anything necessary to keep her remaining daughter Maya safe (the eldest one, Mia, had been already killed), up to and including Morgan's death, or her own. The latter comes to pass when she deliberately channels Dahlia's murderous spirit so little Pearl won't be able to, and "Dahlia" gets killed while trying and failing to bring her plan about.
Morgan might be a subversion. For perspective, Pearl is like eight, and heavily sheltered (she's never left the village, for any reason, ever). Also, Morgan wants Pearl to become the master now. So it's pretty clear that Morgan only wants this not because she feels Pearl is particularly deserving, but because Pearl isn't even old enough to be up after dark, meaning she'll be the puppet through which Morgan (who is somewhat bitter over her position as part of the branch family) forces her own agenda upon the family. Not so Mama Bear. Maya, by contrast, is a much more nurturing caretaker while Morgan is imprisoned, encouraging Pearl to have fun and in general be a little girl while she has the opportunity; Pearl, for her part, reveres Maya completely and would probably flip her lid if she knew what her mother was really up to.
Umineko no Naku Koro ni does this in its first three arcs. In episode 1, Natsuhi Ushiromiya banishes the two remaining servants, the family doctor, and her niece Maria because she suspects them of being murderers and wants to protect her daughter Jessica. This does not end well. She also fights the Golden Witch Beatrice in a duel, although you might call that a subversion: the desire to protect daughter and gun not equal to ability to defeat 1000-year-old sadistic witch. The trope name itself is referenced during the proceedings.