Ascended to Carnivorism

Sometimes, with Carnivore Confusion, all animals are omnivores. Sometimes, only the evil characters eat meat; herbivores eat plants, and "good" carnivores go vegan or eat bugs/fish. Sometimes, carnivores eat meat, herbivores eat plants, and there's nothing anyone can do short of replacing their digestive system... but in the last case, one of the herbivores, a prey animal, might get upset... might get a little obsessive about the injustice... might "crack" a little. Followed by the terrifying (or hilarious) sight of a wildebeest trampling a lion to death and feasting on the corpse.

This trope need not be invoked in vengeance or uprising, but does usually affect an herbivore character that is least a little insane.

A subtrope of Carnivore Confusion. Contrast Vegetarian Carnivore for when a normally meat-eating animal decides to switch to a vegetarian diet.

Anime & Manga

 * In Tales of the Dark Forest, a half-crazy unicorn decides that if he eats the entire corpse of a human, he can gain human powers, including a digestive tract that can handle meat.
 * The Sheepwolf from Kirby of the Stars.
 * In Pet Shop of Horrors, a breed of rabbits go about attacking and devouring people.Only slightly chilling, yes.
 * And the other species these rabbits take after, the aphid, is herbivorous as well...
 * The last part of Chirin no Suzu has Chirin turn into a demonic, flesh-eating ram upon becoming an adult.

Film

 * In The Wild, the wildebeest villain has decided to make himself the top of the food chain. By eating a lion, no less.
 * And of course, don't forget the Killer Rabbit from Monty Python and the Holy Grail.
 * Finding Nemo ends with the little green fish from the sharks' meeting gobbling up the Anglerfish.
 * We're Back! A Dinosaur's Story apparantly features a Triceratops named Woog, who for some reason actually likes to eat hot dogs.
 * Science Marches On. Some paleontologists believe Triceratops was an omnivore, but there's the obvious problem that Woog looks nothing like an actual Triceratops and looks more like a pig with a frill. Oh, and three horns.
 * The second Happy Feet movie featured a krill who was tired of being at the bottom of the food chain and wants to "eat something that has a face".

Literature

 * The creation story in Watership Down has all animal species start out as grass-eating herbivores, some of which were transformed into carnivores by Frith (God) to keep rabbits' numbers in check.
 * The rhinoceros from James and the Giant Peach.
 * In Discworld, the Goshhawk is a special kind of bird of prey, "always on the lookout for the vegetarian option''.

Mythology

 * In the Just-So Story The Deer and the Panther Share a House, after Panther brings home the leftovers of a dead deer, Deer goes out and (with the help of an anteater) hunts down a panther for dinner.

Religion

 * In The Bible, all creatures were originally herbivorous, making all meat-eating creatures, including humans, examples of this trope.

Tabletop RPG

 * Traveller The New Era supplement Aliens of the Rim: Hivers and Ithklur. During a military campaign between the herbivorous K'Kree and the Ithklur, the fighting became so bitter that each side started performing atrocities on the other, including the K'Kree eating dead Ithklur bodies.

Video Games

 * In the bizarre Dreamcast Survival Horror game D2, protagonist Laura discovers that  that was found preserved in a glacier...inside the stomach of a wooly mammoth.

Web Comics

 * In Digger:
 * A possible subversion; Digger the wombat is forced to do this to become an honorary member of the Hyena tribe, but is horrified by the idea.
 * The hyena tribe also has an inversion of this in one of their origin stories. In hyena mythology, hares were originally carnivores and descended to herbivorism.
 * Kevin and Kell:
 * Sort of happened in when a dimensional traveler who was "furrified" into a rabbit on arrival maintained her omnivorous diet. This caused some shock to the local herbivores when she killed and ate a panther that attacked her.
 * And of course, Coney the Killer Rabbit, justified as she's half-wolf.
 * Inverted by Bruno, a wolf who turns herbivore and even has a suitable digestive tract surgically installed. He also later gets herbivore teeth implanted after a fight with Rudy costs him his carnivore teeth.
 * In Sluggy Freelance, the cast always makes sure Bun-Bun doesn't get hungry, fearing he might say "What the hell, let's give meat a try."

Web Original

 * This shirt and the accompanying description is not about retribution or turnabout, but is worth noting.

Western Animation
""Now I've had my head in an elephant, a hippo, and a giant sloth.""
 * The Simpsons:
 * In Helter Shelter, the retirement home residents are watching a nature show about elderly animals. An elderly lion runs out of energy, whereupon the zebra turns around and takes a bite out of it.
 * The Simpsons also sometimes portray koalas as savage carnivores, often gnawing on bones during panning shots of a zoo.
 * Same can go for Elephants, in which they've been depicted as grabbing people to eat them whole. One exception being Stampy, who only puts them in his mouth.


 * A Looney Tunes short in which Sylvester and Tweety are Snowed In featured a mouse who hasn't eaten in so long he "forgot what food looks like." He takes one look at Sylvester and decides to have him for dinner. The same plot was reused later in a Pink Panther short.
 * An unusual human variation occured in Mr. Meaty in which a group of soy-centric vegetarians were supposed to eat their new employee, as part of a plan that even they didn't really understand. Though they did have a nice chart.
 * The Mickey Mouse short The Worm Turns is actually about Mickey Mouse inventing a spritzer that can cause prey animals to turn against their respective predators: A fly attacks a spider, a mouse attacks a cat, a cat attacks Pluto, and finally Pluto attacks Pete the dogcatcher.
 * The Santa's Reindeer in Family Guy had started eating elves and now tore apart any elf that wanders near them.

Real Life

 * Many herbivorous animals aren't above nibbling on sources of meat if it's available. Deer have been documented swallowing small fish found stranded in dried-up mudholes, and hippos will scavenge on the carcasses of other animals (their own species included). As meat is easier to digest than leaves, even an animal ill-equipped to hunt prey is unlikely to pass up fresh protein that's just lying there.
 * Here are some otherwise strictly herbivorous animals that have nibbled on sources of meat if it's available:
 * Cattle
 * Hares and pikas
 * Hippos (see above)
 * Red deer (see above)
 * Manatees
 * Elephants (see below)
 * Even Galapagos tortoises
 * A rogue Asian elephant had killed 17 people in India. Once it was killed, DNA tests on the contents of its stomach revealed that it had consumed human flesh.
 * Some industrial producers of meat feed meat animals...well...meat. Usually they are "animal byproducts" that people wouldn't eat if you paid them. Unfortunately, the inclusion of cow neural tissue in cow-feed is responsible for Bovine Spongiform Encephelopathy (BSE), AKA Mad Cow Disease. This is why we can't eat cow brains, which were traditionally consumed (and considered delicious) by many cultures (and not just ones noted for liking offal—the Midwest's fried-brain sandwich used to be made out of cow cortex).
 * In evolution, animals have switched from herbivore to carnivore countless times. For example, some species of ornithischian dinosaurs probably included some meat in their diet, like the pachycephalosaurs and heterodontosaurs. There's even a new theory that freaking Triceratops ate some meat from time to time. Yikes.
 * In a survival situation, horses and mules can get hungry enough to eat raw meat.
 * Regardless of diet, most placental mammal mothers eat the afterbirth after bearing young, to reclaim valuable protein and minerals their bodies will need to sustain nutrient-costly lactation.
 * In this video a cow actively pursues and devours a live baby chick.
 * Duikers.
 * According to a recent study, Galapagos Tortoises (yes, the beloved and humble Gentle Giant of reptiles) will take advantage of their relationship with Galapagos finches and eat them for protein. To do this, they rise and extend their limbs so that the finch could look for parasites to feed on, only to be crushed to death when the tortoise withdraws its limbs.
 * Pigs are an example of Ascended to Omnivorism, by the standards of their order (even-toed ungulates). As mentioned above, there are other herbivorous ungulates who will nibble at available meat, but many suids are straightforward omnivores.
 * As mentioned above, some dinosaurs that were originally thought to be all herbivores (besides Ceratopsids and Microceratopsids) are actually omnivores, including Pachycephalosaurids and Micropachycephalosaurids.