Truth in Television. Most animals don't give a damn how we classify them, and will gladly eat other animals that are under similar classifications. Raptors eat smaller birds, fish will eat fish, and yes, most bird will peck whatever left over food you leave lying around, even chicken. Mammals eat other mammals all the time. Most humans eat pork and beef, which are both mammals similar to humans.
Chicken farmers have to check the hen houses every day because if a egg sits too long and doesn't hatch, the hen will eat it, and like the taste so much she will eat every egg she lays from then on in.
The "carnivores are mean" subtrope is so damn pervasive, wildlife centers and nature magazines are often deluged with calls and letters from hysterical bird lovers (really?) asking what can be done about the mean hawk eating all the poor little house sparrows. Seriously. For the record, backyard hawks and cats (Cats Are Mean too) mostly go after Eurasian House Sparrows anyway. House Sparrows, for those not in the know, are an invasive species in the Americas who have wreaked havoc on native species. Fortunately they're so used to living around human houses for safety that they're almost too easy for predators to hunt.
Speaking of backyard birds, this article, recently published in Audubon magazine, theorizes that one painting brought the Blue Jay of all animals under the Carnivores Are Mean banner. This isn't anything new for Corvids (poor crows; they're like the hyenas of the avian world), but it's weird that the one member of the family generally agreed to be the prettiest is under this big tent too.
Watch any Youtube video showing hyena or lion predation and see the dozens of comments from viewers asking why the people with the camera didn't save the prey animal from the cruel, vicious predators.
Paleoartist Mark Witton has brought this subject up for discussion in his online portfolio/blog. The discussion, which has been rather lively so far, accompanies his admittedly surprising illustration that depicts carnivorous scavenging behavior in ceratopsians. Even though he explains the thinking behind the piece, several posters can't quite wrap their heads around the idea of a meat-eating Styracosaur — which just underlines his point.
Have you ever heard of David Pearce? The Abolitionist Project? This Tetrapod Zoology post will explain all. Or try to.
"Mad Cow disease" became the issue it was, in the UK, thanks to the ground up remains of other animals being included in the feed for other cows, without the treatment that European feed got that destroyed the responsible prions.
For that matter, some types of animal feeds have warnings about not feeding to other animals because of certain by-products.
What is in fish food? Fish. Or at least, the parts that don't get sent to the shops or restaurants. This conveniently explains why it's so common for cats to sometimes eat the stuff.
Then again, the food pyramid tends to be a lot taller in the sea than on land, allmost anything as large as a small fish or bigger is carnivorous.
Perhaps the closest Real Life example not involving is the case of olive baboons, which sometimes play with baby vervet monkeys as infants. When fully grown, mobs of male olive baboons are quite happy to catch and eat the vervet monkeys who'd been their childhood playmates.
It's kinda the same with people who live on farms. Children sometimes play with the chickens and lambs and such...and then they get eaten. Some are digusted/horrified at this, so refuse to eat them, which is normal.
Chimpanzees enjoy eating red colobus monkeys, which (at least from a human perspective) seem similar to each other. In reality, of course, humans and chimps are a lot more closely related to each other than either is to a colobus or any other monkey (properly so-called).
It isn't unheard of for a predator to become attached to another animal they're supposed to eat naturally. This usually doesn't mean they swear off eating the species, just the individual.
Cows, deer, elk,pandas, elephants and hippos have all been observed and in some cases even filmed eating meat, which in the case of hippos sometimes included other hippos. Very few animals are 'strict' carnivores or 'strict' herbivores. Almost all animals will snack on something outside their comfort zone every now and then.
Polynesian dogs were used as food sources and were fed a vegetarian diet, as this article explains about the Hawaiian Poi Dog.
How many of us have seen a BBQ restaurant with happy pig mascot, merrily serving up his kin on a bun for our dining pleasure?
Try a BBQ restaurant with a happy live pig as its mascot. The pig lying content outside while its owners cook others of its species inside.
Paul the Octopus, just imagine how would it feel being well-kept in captivity when many individuals of your keepers' species happily eat other members of your species as a delicacy, and on top of that, half humans want to protect you and side with you and the other half want to cook and eat you for correctly "predicting" their team's loss in the FIFA World Cup 2010. The Spanish government even promised that should Spain emerge victorious of the World Cup final (as "predicted" by Paul the Octopus) the paella dish would not be served with octopus for a month.
While adult humans are (almost) immune to Carnivore Confusion, most children are horrified when they first learn (or figure out) where meat comes from. Even adults are not completely free from it - people tend to be squicked out by eating animal species that are, in the given culture, usual pet species or otherwise held in high regard. That's the reason people in the West often think that eating cats, dogs, parrots, small mammal pets, or horses is terrible (though horse is eaten in France, Italy, Finland and other European countries), and people in India often see cows in this way, while both believe it's perfectly fine to eat pigs, which are at least as intelligent as any of the above, and certainly more similar to humans socially.
When members of a certain species are seen as a part of family, the concept of someone eating them is akin to someone eating your adopted brother, sister, or kids. Universal prey species are generally immune from being thought of in such a manner because they're rarely kept as pets. Livestock such as cattle and pigs have been bred to be less intelligent and more tasty then their wild counterparts. Horses and cattle are exceptions to this in some cultures, but other factors come to play here - eg. horse eating was banned by Church in medieval Europe because it had a religious significance in certain pre-Christian religions; many European cultures have given up the taboo in later centuries, as the position of Christianity is no longer threatened. Likewise, the significance of the cow in India has a religious basis.
There are semi-vegetarians who classify meat sources by how intelligent they are believed to be (although how accurate these beliefs are vary, especially as science marches on). There are many people who will eat fish but are squicked out by the thought of eating squid/octopus.
In Western societies, the Great Apes are considered to be highly intelligent, and their close resemblance to humans make the idea of eating them seem to most people almost equivalent to cannibalism. This is far from the case in the regions of Africa where apes originate. There, they are commonly eaten — and referred to as "bush meat" — despite various laws and protective regulations forbidding the practice. One of the explanations of the origin of HIV/AIDS is that it mutated from SIV (Simian Immunodeficiency Virus); and was first contracted through the practice of eating "bush meat", due to the physiological similarity between humans and apes.
It's not unheard of for humans to end up in the "bush meat," too.
Quite a few African cultures had cultural taboos against eating chimpanzees, bonobos, or gorillas. However, with all the wars, droughts, and economic depression that Africa has suffered in recent decades, many people have been forced to choose between either abandoning their taboos or starving.