Vegetarian Carnivore

In Real Life, vegetation isn't exactly the best source of food. It's hard to digest, is low in some essential nutrients, and requires a specialized digestive system in order to properly process it into anything useful for the animal eating said vegetation. Because of this, most carnivorous animals are not well-adapted to eating plant materials (unlike herbivores, naturally).

In the world of fiction, however, this isn't the case. A normally carnivorous character can easily switch to a strictly vegetarian diet whenever he/she wants without worrying about digestive problems. This usually occurs in shows with many Funny Animals as a means to address Carnivore Confusion by establishing a normally carnivorous animal as someone who prefers dining on a carrot to dining on a rabbit. It is often used for My Species Doth Protest Too Much characters to indicate that they have switched their diet to separate themselves from their more "predatory" associates. Or, though not as often, maybe they decide that hunting for food is too much work and decide to just switch to veggies.

A subtrope of Carnivore Confusion and Artistic Licence Biology. Contrast Ascended to Carnivorism for when a normally herbivorous animal decides to eat meat.

Note: This only applies to carnivorous, and sometimes omnivorous characters that switch to eating just fruits, vegetables and other plant-based foods (and the occasional fish and/or insect). Carnivorous characters that only switch from one type of meat to another are subversions.

Anime and Manga

 * Pretty much all Pokémon are shown to love eating berries (mostly because they also help heal various status effects in-game). This even includes what should be strictly carnivorous Pokemon like Mightyena (a hyena), Liepard (a leopard), and Arbok (a cobra). The Aerodactyl, Omanyte and Kabuto lines are specifically stated to be carnivorous by the PokéDex, but they are no less eager to eat the berries in the games.
 * Subverted with Seras Victoria in Hellsing. She at first tries not to drink blood and even tries to eat soup at one point. However, doing so proves to be far too painful for her (It turns out it's very painful for a vampire to eat something other than blood) and she ends up feeding on blood anyway (though, mostly from blood packs rather than from live prey).
 * Several of the carnivores including the hero in Kimba the White Lion became this trope when they are no longer allowed to eat each other under Kimba's new rule.
 * Heart from You Are Umasou falls under this being a Tyrannosaurus Rex raised by a family of Maiasaura. It's later subverted, in that he does switch to a carnivorous diet (Though he is horrified at first that he actually likes the taste of meat), but he will still eat berries now and again.

Fairy Tales and Folklore

 * Subverted in a fairy tale where a little girl befriends a wolf she saved. The wolf tries to switch to a vegetarian diet (due to the girl not liking that he has to kill other animals to feed), but nearly starves himself in the process and ends up going back to his carnivorous diet.

Film - Animated

 * Tiger from An American Tail is a vegetarian cat who is appalled at the very thought of eating mice. He does occasionally eat fish now and then, though.
 * Another subversion would be Chomper from The Land Before Time. When faced with the dilemma of being friends with sentient dinosaurs that are also potential prey he decides to switch his diet. Not to veggies, but to insects.
 * An interesting variation occurs with the various pterosaurs ("fliers") in the films such as Petrie. Petrie is a Ptranodon. Pteranodons' diet consisted mainly of fish. However, Petrie and his family are vegetarians, yet it's never stated why.
 * Subverted with Ruby and Strut, which are both plant eating theropods, but since their species were omnivorous in RL it's not particularly odd.
 * Omnivores still need the nutrients that come from meat, and real-life theropods would have starved on the diet Ruby and Strut were eating.
 * What about Elsie the Elasmosaurus? She's basically portrayed as a 'swimming long neck' and has the dental equipment to match. Despite the fact that Elasmosauruses had needle-like teeth and ate mostly fish and squids. Despite the fact that a somewhat more realistic one was shown in the direct predecessor to the film she was in.
 * Though note that she's not actually shown eating anything...
 * Finding Nemo has a trio of sharks (led by a great white shark named Bruce) that want to give up their fish-eating ways and become vegetarians. It's not made clear what they decide to switch their diet to, but it is shown in the film that they are struggling to make the change.
 * Its heavily implied they're eating dolphins. Which makes sense, when you consider that sharks are themselves fish and a "vegetarian" shark eating mammals makes as much sense as a "vegetarian" human eating fish or shellfish; it's possible they gave up eating fish because they're too closely related for moral comfort. But then it makes no sense again when you consider that most extant non-shark fish belong to the class Actinopterygii, which is more closely related to tetrapods like dolphins than cartilaginous fish like sharks.
 * The DreamWorks film Shark Tale had Lenny, a shark that decided to go vegetarian (even the very thought of eating meat made him nauseous). Ironically, he disguised himself as a dolphin (despite dolphins also being predators) in order to prevent other fish from fearing him.
 * Which is also a case of Somewhere a Mammalogist Is Crying since dolphins are just as vicious fish-hunters as sharks, if not even more so. Some dolphins even eat sharks.
 * Inverted in the Disney film The Wild. A herd of wildebeasts are tired of always being hunted by predators, so they decide to switch their diet to meat and become predators themselves.
 * Baloo from Disney's version of The Jungle Book really seems to enjoy dining on fruits and the occasional insect rather than on meat. Justified, in that Real Life bears are actually omnivorous, and plants do make up a large part of their diet.
 * Most bears anyway. The polar bear is a carnivore due to a lack of plants in the Arctic, while the panda bear is a herbivore despite its digestive system being more suited to eating meat, for the simple reason that plants are incapable of running away when you attack them. However, Baloo is clearly neither of these.
 * While he doesn't look like it, Baloo is a Sloth bear. To a degree, this portrayal is quite Truth in Television: Sloth bears primarily eat insects (mostly termites) and supplement with fruits some parts of the year.
 * Simba from The Lion King turns insectivore at the behest of his two animal friends. Just let go of your worries, and you too can grow big and strong on a morally-appropriate diet of creepy crawlies!
 * Subverted with Alex The Lion from Madagascar. He switches his diet to avoid eating his friends, but to fish instead of veggies.
 * The Seventh Brother, a film about a group of rabbits adopting a puppy, actually averts this. While Tiny tries eating the same plants the rabbits do for a while, he begins to grow sick and weak because he just can't live on the stuff. In the end, the rabbits return him to his human owners, and his health immediately begins improving once he starts eating dog food.

Film - Live Action

 * Godzilla Junior from the Heisei (1984-1995) era of Godzilla films is shown to dine on leaves despite the fact that it was established that Godzilla's species is carnivorous.
 * Zilla is something of an inversion. Marine iguanas are strictly vegetarians dining on seaweed, but Zilla's diet consists mainly of fish.
 * The titular character from Theodore Rex is a Tyrannosaurus and a "recovering carnivore". His diet mainly consists of vegetables and cookies.
 * Seven Pounds has Emily, the female Love Interest, claim her great dane is a vegetarian, eating only tofu and steamed broccoli. Obviously, the dog disagrees, devouring meat when offered it by Ben.

Literature

 * The titular giant in The BFG. While his species is carnivorous, he has decided to eat a nasty-tasting vegetable called a "Snozzcumber" instead since he finds the idea of eating children appalling. Interestingly enough, it's the only other option he has besides eating humans since no other plants grow in Giant Land.
 * Subverted in Twilight. The vampires in question claim to be "vegetarians", but they really just feed on animal blood instead of human blood.
 * The good-guy creatures of the Redwall series are mostly vegetarian, save the occasional fish or shrimp. This is plausible for the likes of mice and squirrels, but not for badgers, shrews or otters.
 * Arashi no Yoru ni has this as a driving conflict of the plot, because the wolf Gabu is a carnivore but doesn't want to eat his sheep friend, Mei. He tries to avoid eating meat, but finds he can't do it.

Religion

 * According to Young Earth Creationists, carnivorous dinosaurs such as Tyrannosaurus rex were herbivores. When asked what the T.rex's razor-sharp teeth were used for, they replied that they were used for cracking open coconuts.
 * Answers In Genesis states that they either ate things like Watermelons, or had different dental equipment before the fall of man (...long story there).

Web Comics

 * Inverted in Dominic Deegan, where a character who failed to realise the difference between Orcs being herbivores and vegetarians (in this universe they're the former) tried to convince an Orc character to eat jerky to prove that they can eat meat. Vomiting ensues.
 * Deconstructed in Kevin and Kell, where Bruno (a wolf) needs to undergo surgery in order to receive a digestive system optimized for a vegetarian diet. This is treated in universe in much the same way that gender reassignment surgery is treated in Real Life - some people don't care, some people are happy for him, and a few people think it's strange.

Western Animation

 * Parodied in an episode of Futurama. An Animal Wrongs Group claims that they have taught a lion to become a strict vegetarian. The lion in question looks very sickly and let out a single pathetic-sounding cough.
 * Count Duckula is a vampire duck that prefers to suck the juices out of veggies rather than drink blood, probably due to the summoning mishap where tomato juice was used instead of blood. His servant can't even make him eat meat.
 * Subverted with Marceline from Adventure Time. While she does prefer to eat shades of red (It's implied her entire species can do this), she has no problems drinking blood (Such as when she tried to drink Jake's blood to turn him into a zombie).
 * An episode of The Grim Adventures of Billy & Mandy featured an old lady and her pet spider. Said spider was shown to love eating melons.
 * Parodied in an episode of SpongeBob SquarePants. In the episode, the Flying Dutchman reveals that he devours people when they are shown to be lousy members of his crew. Spongebob remembers that he still has one wish left and wishes that the Flying Dutchman was a vegetarian and therefore wouldn't eat them. The Flying Dutchman then just turns them all into fruit so he can eat them.
 * The Smurfs are shown to be mostly vegetarians in their cartoon show appearances, but have on occasion eaten meat-based products. Though it does make you wonder where the meat actually comes from.
 * In an episode pf Tiny Toon Adventures, Buster was able to train a baby Tyrannosaur to live on a vegetarian diet; he was kind of surprised he was able to do so.

Real Life

 * The Giant Panda is a classic Real Life example. It's an omnivore, and has a digestive system better suited for eating meat. However, its main diet consists of bamboo. This is because bamboo is plentiful and easy for the slow-moving panda to obtain as a food source rather than expending all of its energy chasing after prey. Their very inefficient digestion of bamboo makes them even more unsuited for hunting afterwards. It should be noted, though, that giant pandas will eat meat if given the opportunity.
 * A recent video shows a panda eating a cow.
 * A tragic variation occurs in Real Life regarding pets. For some reason, some pet owners believe that it is more humane to switch their cats', ferrets', or dogs' diet to a vegetarian one and even claim that their beloved pet is happier and healthier. Said owners don't seem to realize that cats and ferrets are obligate carnivores are dogs are omnivores that prefer meat and their digestive systems are not suited for a strict diet of veggies. Note that this can and does lead to severe health problems for the pets (especially cats and ferrets) if not done correctly. There does happen to be a company that makes vegan pet (both dog and cat) food that is both meatless and supplemented with the missing nutrients, but it's not recommended that the average pet owner try this without strict supervision of an experienced vet. There have actually been cases where Animal Cruelty officers have seized pets forced to go vegetarian by owners who aren't doing it correctly and in fact starving the animal.
 * In fact, it's especially bad with cats since felines, along with ferrets, otters, polar bears, tarsiers, aardvarks, and Tasmanian devils are by far the most strict meat-eaters of all terrestrial and semi-aquatic mammals (pinnipeds, sea otters, and toothed whales are even more carnivorous). Not to mention that cats can't eat gluten (wheat protein) and it can in fact kill them. While cats do chew on plants occasionally, they do as a stomach remedy and vitamin supplement, not as a source of calories.
 * An inversion occurs with pet iguanas. Inexperienced owners will often make the mistake of assuming iguanas are insectivores like many species of lizard and will try to get them to dine on a diet of crickets and mealworms. While young iguanas may dine on the occasional bug or two, adult iguanas are strict vegetarians.
 * Any human who switches to a vegetarian or vegan diet, since humans are omnivores by nature. However, this is a much lesser example than most of the others, as humans can adapt to almost any diet.
 * The aardvark cucumber relies exclusively on the aardvark to eat it, defecate the seeds and bury it. The aardvark is otherwise an obligate insectivore, it eats the cucumber for its water content. It won't eat any other fruits, though.
 * Recent studies actually show that certain theropodal dinosaurs (whom prior, were originally thought to be all carnivores), particularly Compsognathosaurids, Deinonychosaurids, Troodontids, Oviraptorosaurids, Alvarezsaurids, Therizinosaurids, Ornithomimosaurids, and Archaeopterids, are actually omnivores.
 * The unfortunately named Kinkajou (he he kinky Jew), is a monkey/cat like animal of the rainforest whose diet consists mainly of fruit despite being classified in the order "carnivora" and has sharp teeth.
 * The binturong, or bear cat, is an omnivorous animal sometimes compared to a bear (although they are more related to mongooses), whose diets consist mostly of fruit, but will eat eggs and small animals if given the opportunity.