Unlike humans, animals seem to have an inherent ability to detect a malicious character even if it's not obvious to anyone else. May not apply to pets of a villain, although attack dogs and Bond-style cats have a surprisingly low sense of loyalty in a pinch.
This doesn't always count in a comedic situation; snotty pets frequently hate everybody, and will be nasty to characters who likewise dislike them as well as benign ones who only want to be friends.
A character's dog reacting oddly to their presence is often the first sign that character's been replaced by an Evil Twin. Or just an ordinary twin. This is not actually implausible, unless the scent of the replacement for some reason is exactly the same as the replaced person. Heart rate and other things might reasonably be detectable to some animals, too.
If the animals know something's coming, sometimes they don't run, they go quiet. In fact, too quiet...
This trope is not restricted to dogs - cats and horses are also commonly invoked, amongst other creatures. Nor necessarily animals - some plants have the ability too. When a human character has this power, it's a Gut Feeling.
Compared to Evil-Detecting Baby.
Not to be confused with Even the Dog Is Ashamed.
If the dog can detect the supernatural, it's because of a Glamour Failure.
Contrast Licked by the Dog, where this is used to show the audience a good guy instead of a villain. Then again, sometimes they aren't detecting evil at all, and it's simply that Animals Hate Him.
Closely related to Scare The Dog.
If the dog is attempting to warn its master or another nearby human character but the warning is dismissed (typically due to annoyance at the dog's repeated barking or other unusual actions), it can be a form of Not Now, Kiddo. Animals can also detect especially good people, if they're a Friend to All Living Things.
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Not evil, but in Tokyo Mew Mew, dogs can sense that Ichigo is a Catgirl even when she's in the form of a human, and start barking at her for no reason. With her Lancer Minto and her boyfriend Masaya both owning dogs, this becomes a problem.
Kicchomu in Thriller Restaurant has an uncanny skill in detecting evil spirits and doppelgangers. This is exhibited in episodes 6 and 14.
In Saber Rider and the Star Sheriffs there's the Scottish terrier that belonged to Saber Rider's parents. When they were kidnapped and impersonated by Vapor Beings in "The Highlanders", he barked and hissed at them, which weirded Saber out since he was very sweet to them...
Maru-chan is initially terrified of Sawako in Kimi ni Todoke. This is a subversion, however, since Sawako only looks creepy and is actually one of the kindest people you could hope to meet.
In Fullmetal Alchemist, the Big Bad starting to move around usually coincides with every dog in Central going insane. Not only can animals sense the homonculi and living Philosopher's Stones, but even the gentle dog Den reacted viciously to one to show the inherent wrongness of what they are. Not only that, when the Big Bad exerts his power enough, he can make every single animal in the city go crazy and bark at the earth, not just the dogs. The Big Bad himself makes animals too afraid to even approach his lair.
Special mention should also go to Black Hayate, Riza Hawkeye's dog, who not only can sense the Homunculi but viciously attacked one to save his beloved mistress. Roy Mustang awarded him the honorary rank of Second Lieutenant for his services to their cause.
Kiba's dog, Akamaru, from Naruto is able to sense just how strong a chakra is in a person. When he starts whimpering, that means trouble close by.
The Jinnouchi family dog in Summer Wars is so good at detecting evil that it can sense a satellite changing course in orbit.
He's pretty skilled at detecting good as well, if his reactions towards a certain Jerk with a Heart of Gold are anything to go by.
The two ghost dogs of Kekkaishi are primarily used to detect other evil spirits or people.
In Wild Rose, Tranquilo immediately starts growling when he detects the rogue manbeast's presence and tries to protect Camille from him, getting badly injured in the attempt. In contrast, Tranquilo's immediate acceptance of Kiri shows that he's safe.
The hentaiAlien from the Darkness has Einstein, a ferret who growls and hisses whenever alien-possessed Flair is nearby.
In Princess Mononoke, San and her wolf companion find hundreds of mice scurrying away from the direction where humans using boar skins to masquerade themselves to find the Forest Spirit. Apparently they're so terrifying that they willingly scurry over a full grown woman and a giant wolf to get away.
In episode 15 of Tiger & Bunny Keith Goodman's dog, John, growls at and tries to keep Keith away from the pretty girl on the park bench. The reason being that while Cis looks like a nice lady, she's a Deceptively Human Robot (in appearance if nothing else) and Keith doesn't realise this.
Hotarubi from Basilisk has a pet viper that could see through Kisaragi Saemon's disguise. Unfortunately, Hotarubi didn't understand the meaning of her snake biting her lover and just tossed the animal away and sucked the poison out of Saemon's finger by herself!
Jeudi's cockatoo Printemps from Honoo No Alpen Rosehates Matilda aka the fake Jeudi on sight and openly attacks her, much to the surprise of everyone else. The "victim" even lampshades the trope via asking if Printemps thinks she's a bad person. He was right.
In a milder version, he also dislikes Leon who is far from a bad guy, but is one of Jeudi's suitors who can potentially take her away from her boyfriend Lundi.
Yondemasuyo, Azazel-san has dogs and cats, and pets, and er, hobos, apparently, in the first episode of the second season. Although not in response to the demons; it's to Akutabe.
When Impulse Bart Allen was replaced by his Evil Twin Inertia, the only character to realize was Bart's dog, but Inertia was able to get around that with treats. And by not thinking any evil thoughts around the dog.
In Supreme Power, his foster parents give the alien baby Hyperion (a modern version of Superman) a puppy for his first birthday. It immediately senses, that he is not human, and tries to attack him. He promptly (and instinctively) annihilates it with his heat vision.
Wookiees are explicitly sentient, but since they're hairy and unintelligible, unimaginative writers of books and comics alike love to make them as animalistic as possible. This is taken to a new height(or low) in one arc of the X-Wing Series - a woman who has a Wookiee hanging around due to a life-debt doesn't understand him, and he doesn't communicate with her at all, he's just a bodyguard - who starts growling and provoking "Groznik senses something!" before they get attacked.
One Far Side comic strip had a dog preparing to growl at an empty closet, just to mess with his human owner's head.
In Zombies: A Record of the Year of Infection the main character writing an Apocalyptic Log notes that dogs go nuts at the presence of zombies, including frequently attacking them. Furthermore his dog seems to mistrust the people when he reaches the supposed "safe haven" of Churchill in Manitoba Canada. Since there is No Ending, exactly what happened is left open to interpretation, but one interpretation is that the highly isolated and suspicious locals killed the main character.
The demons in Chick Tracts are invisible to all the characters... except for Fang, a (badly drawn) dog who occasionally barks at them in the background.
In the mid-90s RoboCop Versus The Terminator comic book, one of the humans remarks on the fact that a dog acts friendly, rather than hostile, to Robocop, proving to the human that said character couldn't be a Terminator.
Batman tried to bring up this trope in the Superman/Batman comic, The Supergirl From Krypton, noting how Krypto hated Kara Zor-El, Superman's cousin who had just arrived on Earth. It was ultimately subverted as Superman pointed out that 1) It was Krypto's job to protect the Fortress of Solitude, and 2) He hated everybody.
The cats are used for this in Aeon Entelechy Evangelion, as they are sensitive to anything abnormal, a quality shared only by human infants, and thus are also used in CATEYES security systems. This also seems to be common knowledge since Shinji notes their absence in the dome where Rei lives.
In The Dilgar War dogs are the only thing that can locate the Spectre, Dilgar special forces equipped with cloaking suits.
Invoked in Nutss Ghost Story when Murphy tries to convince Molly that Shepard's alright by reminding her that Mouse hasn't had his usual reaction to Outsiders around Shep.
In Blue Sky, even though Wheatley is not really evil at that point, Duke can still detect that he isn't human.
Lucille Harewood's dog in the Star Trek fanfic Safe and Sound was specially trained for this purpose because of what her father did for a living. This is how her mother figures out that Dr. Korby isn't as trustworthy as he seems...
Simultaneously referenced and used straight in Ace Ventura. When Ace is making the acquaintance of Roger's dog on the crime scene, the dog barks at Lois Einhorn when she walks in:
Ace: "So, animals can sense evil."
He later learns that Lois is the killer — and as it turns out, a man. And if the dog witnessed the murder, his reaction to Lois, someone who harmed his master, is perfectly natural.
Shortly after he has been attacked by a werewolf in An American Werewolf in London, all sorts of animals start hissing and growling at David. The only exception is wolves. They don't seem to mind him too much.
In Arachnophobia, both a cat and a dog can sense that the General is, if not technically evil, than certainly not to be messed with. A crow does not realize this in time.
In Cello, the family's pet dog, Sunny, barks loudly when it is near the titular musical instrument.
Lester the family dog barks at Michael Myers hiding behind the bushes in Halloween (1978).
In the film version of Hannibal, Hannibal Lecter breaks into Agent Krendler's house and his dog is initially hostile, but Hannibal actually makes the dog cower in the corner while he pokes around.
In Disney's The Little Mermaid, Eric's dog absolutely loves Ariel. However, he hates Vanessa (Ursula's human alter ego) and growls at her, giving her a literal Kick the Dog moment.
In The Lost Boys, Nanook growls at Michael, after he drank some of the blood-in-a-wine-bottle while hanging out with the vampire hoodlums.
In Near Dark, the earliest sign that love interest Mae is not all she seems is that the hero's horse goes nuts when he tries to show him to her.
In both the 1961 original and the 1998 remake of The Parent Trap, the dog owned by one of the twins reacts with barking and growling when the other twin, pretending to be her sister, tries to greet him.
In Sleepwalkers, the villains morph into somewhat feline creatures, but are hated by actual cats.
However, Terminator 2: Judgment Day includes a scene where a dog does not react to the T-800. The filmmakers admit this is a plot hole, but felt it was better to not confuse the audiences.
It may be due to the fact that the T-800 is more or less benevolent, if aggressive as hell towards its (or John's) enemies. It doesn't really care about much beyond John's safety. May also be why the dog didn't regard Robocop as evil in the Robocop vs. Terminator comics. If the machine itself is benevolent, the dog may pick up on it the same way it detects evil and malevolence in all other instances of Terminators.
Backing this up, the dog does bark like crazy off-screen at the malevolent T-1000 while it's impersonating John's foster mother.
In Hook, Nana the dog senses that the titular Captain is about to invade the Darling home and starts barking.
In the short film adaptation of Stephen King's The Road Virus Goes North, the protagonist is dumbfounded as to why dogs bark madly at him wherever he goes. They're actually disturbed by the cursed picture he's got in his car. When the titular Road Virus himself first appears, the Evil-Detecting Dog falls instantly silent except for a few terrified whimpers.
Also used in the Stephen King miniseries Rose Red, where an otherwise nice dog bites the powerful, autistic psychic Annie. While Annie is not exactly evil, she does retaliate by causing rocks to fall from the sky and crush the house of the people who own the dog, so the dog's attitude was arguably justified.
In the film Alien, the cat is able to detect the titular creature, though no-one apart from Ripley seems to be aware of this.
Brett does notice that the cat is hissing at something, but by the time he turns around to see what's bothering Jones it's really too late.
In Aliens, during the opening dream sequence, Ripley's cat senses the Chest Burster inside her and starts to hiss.
In Alienł, a dog barks incessantly when the xenomorph is present.
In the teaser for the H.P. Lovecraft Historical Society's adaptation of The Whisperer in Darkness, a scholar listens to a wax cylinder recording of a man spouting occult ravings while a cat sits impassively nearby. Suddenly ANOTHER voice cuts in on the recording, and the cat startles and hisses.
In In the Line of Fire, Pam's dog growls aggressively at "James Carney", who is really would-be Presidential assassin Mitch Leary, and who kills Pam and her roommate.
In the obscure Spanish film Nocturna, all the cats in the Cat Shepherd's pack start to go and yowl at a wall. The Big Bad of the film, the Shadow, is then revealed, slipping out of the, um, shadows.
In Cloverfield, when the trio head into the subway system to hide, they find dozens of rats scurrying away from the rear end of a darkened tunnel. One of the characters, Marlene, is Genre Savvy enough to realize they have to get away from the area the rats were running away from. Minutes later a giant parasite attacks the group and injures Marlene, who dies a nasty death from the infected bite.
Super 8: All the dogs in the area run away to other towns because they can tell there's a monster running around.
The Lord of the Rings: The Horses have every reason to be scared of the mountain where the Dead live. For once, the Humans understand the reasons for their fear. Elves, however, don't. The film specifically inserts a scene in which a Hobbit's dog cowers and retreats in fear from a Ringwraith.
Kelly's dog Ginger in Race With the Devil is the first to notice the cultists. It later gets hanged from the RV's door.
Used a lot in the 1944 film The Mummy's Ghost, in which dogs barking the alarm or biting at his ankles actually pose a greater obstacle to the mummy's comings and goings than anything the human characters do.
In John Carpenter's The Thing (1982), the dogs in the kennel definitely know something is off with the newcomer...
In M. Night Shyamalan's Signs, the dogs smell big-time alien and bark and bluster accordingly.
In Jeepers Creepers 2, the farmer's dog is the first one to notice that there's something off with one of the scarecrows...
The president's dog in Mars Attacks! barks at the Martians on the television.
In Epic Ozzie growls at the bat call, which is being sung by Mandrake.
The female character's pet dog in The Hole (2009) runs away when the human character's are investigating the, uh, hole and she states the dog doesn't like it. The dog also freaks out when something tries to drown the little brother.
Averted in Perfume. Thanks to the villain's complete lack of scent, he steps right over a sleeping dog as he's sneaking up to Laura's bedroom, without it sounding the alarm.
Van Helsing in Dracula (1979) confirms his suspicions about his late daughter's vampirism by investigating her resting place with a horse. When the horse comes by her grave, it stars stomping the ground with its hoof, and then runs away in terror. Van Helsing and Dr. Seward then start digging.
In Muppets Most Wanted, Animal realizes right away that Kermit has been replaced by an evil criminal mastermind.
Animal: Bad froggy!
Outpost: Rise of the Spetsnaz. After being thrown into a cell in an underground Nazi medical experimentation lab, the Russians note that despite the smell of decay there's no sign of rats or their droppings. "You know you're in trouble when the rats have abandoned ship."
More like "Evil Detecting Horses", in this case in The Wolfman (2010). Also evil detecting bear and evil detecting deer. Ironically averted by the single one dog (it always growls at Lawrence, well before he is infected, while it is completely oblivious to the original werewolf because he is its owner).
The Perron's dog Sadie in The Conjuring refuses to enter the house, sensing the evil force within. Unfortunately, this doesn't save her.
Sweeney Todd in A String of Pearls: A Romance, a pseudonymously written penny dreadful serial, is hated by a dog belonging to one of his victims. This actually forms a key part of the plot.
In the sixth book of Jim Butcher's series The Dresden Files, Blood Rites, Harry comes into the possession of a puppy, who he names Mouse; during it, he growls at people that Harry later learns are evil. Mouse grows up to be a very, verylarge dog, and is explicitly identified as a Temple Dog: the descendant of a spiritual creature who bred with a normal dog, with a couple of supernatural abilities, mostly of the detection or warning variety. He's also way smarter then a normal dog, occasionally looking like he understands English to a degree. In Changes, we find out that he can. Mouse's breed is in such high regard that Ancient Mai, one of the most powerful and nastiest wizards, will accept its judgment and testimony without second thought. Also Nicomedeus, a 2,000 year old demon-possessed arch-villain and leader of a bunch of other demon-possessed people, is apparently afraid of Mouse.
Harry's first pet, a large housecat named Mister, gets along fine with the supernatural... so long as it's not an immediate threat, or else interrupting his dinner.
Funnily enough, while Mister is big, he's tiny compared to Mouse, yet he constantly bosses Mouse around. Anyone who's ever owned a large dog and a smallish cat know that's totally Truth in Television, especially if the cat was able to establish its dominance while the dog was still a puppy.
The protagonists' pet dog in the first Goose Bumps book, Welcome to Dead House barks around everyone in town since it can tell that they are actually dead. Eventually, the dead people get fed up with it and actually kill the dog as a result.
Likewise in Ghost Beach, where the old man's dog ultimately detects that the kids' aunt and uncle are ghosts too.
In Samuel Taylor Coleridge's "Christabel", one of the first signs something is not right with Lady Geraldine is the untypical reaction of the dog: "The mastiff old did not awake,/ Yet she an angry moan did make!/ And what can ail the mastiff bitch?/ Never till now she uttered yell/ Beneath the eye of Christabel."
In Joseph Sheridan Le Fanu's Carmilla, the mountebank's dog will not go near Carmilla and howls continually in her presence. Possibly a nod to "Christabel".
"Silver Blaze", by contrast, famously includes "the curious incident of the dog in the night-time" - the curious incident being that the dog did not bark at an intruder, thus alerting Holmes to the fact that the dog knew the intruder, therefore the crime in question must have been an inside job.
Pretty much universally true in H.P. Lovecraft's works. (Below are just a few examples.) On the other hand, humans have almost the same ability, as Lovecraft's narrators and characters constantly go on about how eldritch and wrong the things they encounter feel (or seem or look or smell or sound), even if they can't explain why or don't even clearly see anything.
In The Dunwich Horror, the weird (and unnatural) Wilbur Whateley carries a gun for self-defense against other villager's dogs. A dog still gets him in the end. In a library, of all things. And just to show just how much dogs hated him, it actually crashed through a plate glass window in order to attack Wilbur.
In At the Mountains of Madness, the expedition's dogs go nutzo over the strange beings they found frozen in a cave under the glacier. PROTIP: they're aliens (kinda) capable of extremely long periods of hibernation.
Lampshaded in Rats in the Walls, though it was a cat rather than a dog.
"I realize how trite this sounds- like the inevitable dog in the horror story which always growls before his master sees the sheeted figure- yet I cannot suppress it."
The Starks' direwolves in A Song of Ice and Fire will growl at anyone their masters don't trust, and in at least two instances, someone they trusted but really shouldn't have.
Jon Snow's direwolf Ghost was also able to figure out that something was wrong with corpses they had found beyond the Wall after all other animals would not approach them.
Bran only trusted Sam Tarly after Summer, his direwolf, licked him. Sam remarking that he was friendly with Jon Snow's Ghost helped, too.
In the Discworld series, one of the only creatures capable of seeing Death when he's not trying to be visible are cats. Strangely, Death actually likes cats, and in fact one of the things he hates most of all are people who mistreat cats.
In Sourcery, the Patrician's dog, Wuffles, growls at the sourcerer's staff.
In J. K. Rowling's Harry Potter novels, the cat-like creatures called Kneazles explicitly have this ability. Crookshanks is part-Kneazle. When Ron realizes that this is the reason Crookshanks hated Scabbers so much, he holds his new owl in front of Crookshanks to test his reaction; Crookshanks's lack of interest is good enough for Ron.
Also potentially of note is that spiders are afraid of basilisks. I mean, honestly, if it's so evil that even "spiders flee before it"....
In the book The Zombie Survival Guide, dogs can sense zombies, reacting with maddened rage unless having been born in fully zombie-infested times. Taken further, in World War Z, dogs are specifically trained for zombie-fighting, helping escort special forces teams and scouts, and with smaller, tough breeds, going into areas humans cannot fight safely in.
Although all other animals take the second variety; one way of telling if a zombie is near you is if the wildlife is unnaturally quiet. This comes in quite handy in bogs, rainforests, etc., but applies everywhere.
Also in World War Z, one safe zone is shown using dogs in kennels arranged along a walkway as a screening process for refugees entering the city. If the dogs go nuts when you walk by...well, sucks to be you.
In The Lord of the Rings animals react strongly to the presence of the Ringwraiths. In the books, the wraiths' horses (and later fell beasts) are an exception, due to being trained from birth to tolerate them. The first movie features spiders, wetas and other creepy-crawlies moving to get away from the wraiths, although the horses look quite evil.
A dog specific example would be Farmer Maggot's dogs who really didn't like the Ringwraith who came visiting.
Which doesn't really mean much, in the case of those dogs. They're guard dogs, they don't like strange intruders in general. Frodo admits to being terrified of them, due to an incident in his youth when Farmer Maggot caught him stealing mushrooms and ordered the dogs to run him off the property, which they enthusiastically did.
In one book of Mercedes Lackey's Dragon Jousters, the tamed dragons - who are established as being as smart as bright dogs - hiss and go into threat displays whenever they see a Magi. This might just be because they watch their riders' body language and know that the riders hate the Magi.
In The Saga of Darren Shan, cats and dogs hate vampires (and presumably vampaneze) though wolves, bats and rats love them. In The Vampire Prince, the wolves that are gaurding Darren however, know and appear to despise a troup of vampaneze hiding in a tunnel in Vampire Mountain.
In The Bone Doll's Twin, dogs instantaneously dislike Tobin because he had necromancy performed on her as an infant, to appear like her murdered brother.
Subverted in a Father Brown short story. In the beginning, a young man is telling the story of a murder case where, he claims, his dog told him when the murder took place by howling, and who the murderer was by barking at him. Father Brown thinks about the story for a week, and then tells his own interpretation to the young man. This interpretation is based on having the dog acting entirely as an animal, it points to another person as murderer, and it is of course the correct one.
A Christmas Carol: "Even the blind men's dogs appeared to know him; and when they saw him coming on, would tug their owners into doorways and up courts; and then would wag their tails as though they said, 'No eye at all is better than an evil eye, dark master!'"
In Let the Right One In, cats have an immediately hostile reaction to vampires and attempt to kill them on sight.
In Marley and Me (a true story), Marley goes from being a typical goofy, playful, exuberant Labrador to ferocious attack dog when the author's neighbor is assaulted. Earlier, he again does a 180 in demeanor when he senses the authors and his wife's sadness after miscarrying and quietly rests his head in her lap, making him a Sadness Detecting Dog as well.
The Saga of the Noble Dead features Chap and Shade. As majay-hi, descendents of Fay-possessed wolves, they have the innate ability to sense the presence of undead.
An icy trickle disturbed Conan's spine, and he wondered if, after all, Pelias were a man, or merely another demon of the pits in a mask of humanity.
Tuck Everlasting has cows. Yes, cows that follow a dirt path and quickly go around the forest that the main character flees into. That's one of the reasons why the humans also stay away.
One young adult horror novel (can't remember the name) had a once-human snake woman terrorizing the protagonists. As an unnatural monstrosity, she was such an Enemy to All Living Things, that the zoo's fennecs (tiny desert foxes smaller than domestic housecats) tried to attack her from their enclosure. In the end, the heroes trick her into one of the animal pens and the frenzied animals tear her apart so thoroughly she is unable to regenerate.
In Perdido Street Station, Lublamai's pet badger Sincerity is the first one to detect the threat of the runt slake-moth, growling downstairs as it emerges from its cocoon in Isaac's loft.
Inverted in the Belgariad, birds seem to instinctively know (and love) Polgara, even if they've never met her. The Emperor's bird even chews him out for not recognizing her.
In L. Jagi Lamplighter's Prospero Lost, when Theo's dog accepts Mab's entry without complain, Theo visibly relaxes.
In Red Death, a spinoff of The Vampire Files, the newly-risen and morally-insecure vampire Jonathan Barrett is reassured by how his family's dogs greet him with happy enthusiasm, averting this trope and suggesting he's not become evil.
In Firebird (Lackey), all the animals around the palace hate Tsar Ivan and Ilya's brothers, calling them the Monsters.
In Emergence, Terry (a macaw) takes an immediate and intense dislike to Rollo, an opinion later events show to be entirely justified.
In Coraline, the mice belonging to the crazy old man upstairs give the eponymous protagonist a message by proxy: "Don't go through the door." They're absolutely right — said door leads to the fantastical Other World, which contains magic toys, delicious food, and a life-sucking beldam who just loves children...as in, loves to steal their souls.
Played with in Caitlin Davies' The Ghost of Lily Painter. Jojo the dog reacts with anger or fear to Lily's presence, but aside from some occasional poltergeist-style activities, Lily turns out to be benign.
Not evil per se, but cats, dogs, and wolves in The Wheel of Time instinctively know whether a person can channel the One Power. Given male channelers' tendency to go violently insane from the Dark One's touch, this would come in handy for isolated villages, although the link doesn't seem to be widely known.
E. F. Benson's "The Room in the Tower" features a dog who is very suspicious of Julia Stone's grave. Appropriately so, since she's a vampire. The pet cat, on the hand, can't get enough of the evil vibes.
In The Woman in White, when Sir Percival greets Miss Fairlie's "little Italian greyhound", it whines, shivers and hides under the sofa from him, then barks and snaps at him when he leaves.
Subverted in In the Seasons of Rains by Ellen Steiber. The protagonist hears one of his cats (a female) screaming in terror, but when he goes to investigate finds his way blocked by his two male cats who scratch and bite him. When he returns to his bed, the female cat is sitting apparently unharmed on his bed. The man is being stalked by a Horny Devil (suggested to be the biblical demoness Lilith) who can shapeshift into cats or owls.
Carlos: Something about your friend I should know?
Sarah: Cat person.
Played for Laughs when Cameron goes to a computer convention and a robot dog barks at her.
War of the Worlds: "Amongst The Philistines", Kensington and Mrs. Pennyworth buy a dog just in time to have it react badly to Adrian, a few scenes before he turns out to be an alien. The trope is pretty blatant here, as the dog was introduced in this scene, and is never seen (alive) again.
Happened in The Dukes of Hazzard when Boss Hogg's good twin came to town, and Rosco's dog was used to prevent Boss from stealing his inheritance.
On Dexter (and the novels it's (loosely) based on), animals in general and dogs in particular can't tolerate the presence of the main character (a sociopathic serial killer). It's so bad that when he tried to keep a pet turtle it starved to death rather than come out of its shell when he fed it.
There's a dog that seems rather fond of Trinity in the fourth season, though.
In Reaper, the first sign that The Devil has taken possession of Sam's soul is that dogs suddenly hate him.
In Star Trek, it turns out that tribbles can detect disguised Klingons.
In Star Trek: Enterprise, captain Archer's dog barks when a web-like alien passes in the conduits overhead.
Played with on Heroes. Mr. Muggles always seems to know which characters shouldn't be trusted, but being that he's a tiny Pomeranian, there's not much he can do and he seemed to like a disguised Sylar and Revenge!Ted.
Played more straight on a deleted scene when Sylar has Claire trapped in a closet and Mr. Muggles goes mad barking at him but gets flicked out the doggie door for his trouble.
Not really evil detecting, but in My Own Worst Enemy the family dog is the only one who can tell when its owner's hard-edged split personality takes over.
Invoked in Psych. Shawn is trying to fake a haunting in a house, and uses an electronic silent dog whistle to make a homeowner's dog start barking angrily. He then claims the dog is sensing an evil presence.
In Prue's penultimate appearance on Charmed, she is changed into a dog so she could be able to detect the cries of a Banshee (who, by the way, is a possessed Phoebe) and lead Piper to her.
They also used to have an Evil Detecting Cat which hisses at and has been known to run from evil beings and demons, especially those trying to fool their way into the Halliwell household by appearing mortal.
Invoked once in Supernatural when Sam used this to explain why a dog kept barking at a house where a supernatural event had happened. It was never proved on screen, though.
Subverted in Battlestar Galactica, when Cylon D'Anna is quite pleased that Jake the dog doesn't react with hostility to her. Of course, little does she know that Jake serves La Résistance in another way.
A light version of this appears in NCIS when Kate is reluctant to adopt a dog whose owner has died...until it growls at her Handsome Lech colleague who's always making unwelcome moves on her.
In Tales of the Gold Monkey, Jake's one-eyed dog Jack growls or barks at any impostor, including the (unknown-to-the-characters) resident Nazi spy.
Episode 6 of Buffy the Vampire Slayer, Xander's change to evil is detected by a pig. Justified in that he later ate the pig.
Buffy: He scared the pig! *Giles looks at her* ...Well he did...
Subverted all the time on It Takes a Thief (2005)—the big, mean-looking dog the family owns as a thief deterrent is much more likely to approach the thief in a friendly manner or run away whimpering.
In The X-Files, "Terma", Soviet assassin Peskow manages to upset all the horses in a stable by calmly observing one of them. Though for all we know it was part of his plan.
In The Event, dogs bark at the aliens, who are otherwise indistinguishable from humans
Happened several times in Forever Knight. A dog and a horse both reacted negatively to vampiric presences.
In The Nanny, while C.C. wasn't evil (supposedly,) she was a full-blooded Rich Bitch, and her Pomeranian Chester had no trouble letting her know it. He would snarl and growl every time she was anywhere near him, while being perfectly friendly to everybody else in the house. In real life, despite Lauren Lane (C.C.) being a very sweet and pleasant person, Chester was Fran Drescher's real life dog, and was so fiercely loyal to her any time Lane picked up the dog, he would just naturally growl, so they wrote it into the story that way.
On Once Upon a Time, Dr. Hopper's dog, Pongo, is a well-behaved and sweet Dalmatian. But once Regina, the former Evil Queen waltzes into their office, the dog starts growling at her and gets frozen for his trouble. That's because it's really Regina's mother Cora in disguise as her daughter. Pongo was previously shown being well-behaved around the real Regina.
On The Golden Girls, Blanche has just found her current boyfriend cavorting with Rose's sister when the dog that they've been sitting for comes out into the living room carrying the man's pants. After being told off by an infuriated Blanche, the abashed man goes to get his pants and leave, only to be barked at and chased out of the house by the equally angry dog.
Subverted in Prison Break. The dog begins barking at Haywire and then follows him down the road after he steals supplies from the camping store. It turns out the dog wanted the jerky in his backpack.
Suggested by Loreena Mc Kennitt's version of W. B. Yeats' "The Stolen Child", about mysterious faeries that seduce and steal away the titular child, which begins and ends with a chorus of barking hunting dogs who have detected the sinister faeries.
Alice Cooper sings about how his dog and cat turn on and attack him in the final verse of "No More Mister Nice Guy".
In Tales of MU, dogs can detect the main character's ancestry and are utterly hostile to her. This naturally results in her being afraid of them.
In the Ted the Caver webfiction, a Jack Russel terrier named Trip refuses to go into a sinister cave the protagonists are investigating.
The setting of Palladium's Rifts RPG takes the ball and runs with it; not only are animals explicitly said to be sensitive to supernatural evil, but one faction has genetically engineered anthropomorphic canines ("Dog Boys") specifically to exploit this ability as anti-supernatural police.
All creatures of nature are very sensitive to the presence of all creatures of the Cthulhu Mythos. They instinctively call out their warning sounds and flee if any of the Old Ones or their minions come within range of their senses.
The world at large hates Prometheans, and animals aren't any different. While Prometheans aren't evil, they are soulless and unnatural, and animals can instantly sense this and often move to attack. Of course, there's a whole school of Transmutations that allows them to redirect this rage in creative fashions...
Vampires also have trouble dealing with animals unless they know the Discipline of Animalism, being living corpses. Conversely, if they do know Animalism, they can seem like a Friend to All Living Things.
Reversed in GURPS. The effect comes from entities with the Frightens Animals disadvantage rather than special powers that dogs have.
The Blood Of Heroes RPG. Clint, one of the sample characters, has a dog that can detect evil.
In the Chill game animals (including dogs) can detect the use of Evil Way Disciplines or creatures from the Unknown up to 200 feet away.
This was one of the flaws of being one of the PumoncaChanging Breed in the Old World of DarknessWerewolf: The Apocalypse. As noted in their sourcebook, "All beasts fear the Pumonca. No horse will bear him, no dog will follow him." Apparently every other creature in existence except for humans can catch on to the fact you're actually an eight hundred pound werecougar in human guise and they all respond accordingly. This makes sense since cougars are one of the largest native predators to American soil, doubly so when they are some sort of werecreature, neatly explaining other animals' fear of them—to dogs, horses, and other animals, a large predator is a Very Bad Thing. This means it's less 'evil detecting dog' in most cases and more 'predator detecting dog.'
Hewie from Haunting Ground barks at hidden traps and other unseen dangers, allowing the mostly helpless Fiona to work around the many perils of Belli Castle.
Brown from Rule of Rose works very much like Hewie and can detect and bark at enemies long before Jennifer (or the player) can see them. He can also sniff out items though these aren't exactly "evil".
Not exactly evil-detecting, but at one point in Metal Gear Solid 3 you need to put on an enemy uniform and mask to infiltrate a base. Scientists, guards, and security cameras take you for the real deal and don't raise alarms. But you don't smell like the officer you're impersonating, and guard dogs will still raise alerts.
Also, in Metal Gear Solid, Sniper Wolf's wolf-dogs are unfooled even by your Optic Camouflage. The only way to get past them without hassle is to have one of the puppies piss on your cardboard box, or to have Wolf's handkerchief equipped.
Another not-quite-evil-detecting example, in Assassin's Creed III, the British's dogs may start barking at you and grab the owner's attention, even if you haven't done anything "wrong" in front of it.
Parodied in Kagetsu Tohya. Shiki learns that a black cat (Len) frequents the Tohno mansion courtyard, but only lets Hisui approach it, and he wryly comments that animals really can sense dangerous people. Akiha and Kohaku are decidedly notamused...
In the firsttwo games attack dogs are a soviet unit that can detect spies that can fool any other unit and your base defenses.
In the third game the dogs switch sides to the allies but the soviets one ups this by replacing the Soviet Huskies with armored war bears.
In Red Alert 3: Paradox, you need to use each faction's animal scout unit to detect stealth units, which is not only limited to Evil Detecting Dogs but Evil Detecting Bears, Hawks, Tigers, Sharks, and Giant Robot Crabs too.
In Snatcher, it's commonly believed that animals can detect Snatchers, to the point where genetically engineered "Pocket Pets", which can be used as purses, are becoming extremely popular.
In Mother 3, Lucas, Kumatora and Duster eat some unusual mushrooms while desperate for food. The mushrooms cause them to hallucinate that the monsters in the area are characters from their pasts. The party dog, Boney, is the only one not fooled, and whines and barks when the hallucinations begin to talk to the characters, attempting to warn the rest of the party as to their true nature. He also refuses to get into a beautiful sauna the party can rest in - if you visit it again after the effects of the mushrooms have worn off, you'll find the humans were deliriously bathing in toxic sludge.
In Super Paper Mario, there is a large guard dog-esque monster in Merlee's mansion who is chained up. When released, he runs past Mario and chases off the mansion's maid, who is actually one of the Big Bad's minions, the shapeshifting Mimi.
Despite disguises, dogs in the Hitman series will often, if not always, sniff Agent 47 out.
Hunter pets will also occasionally stop dead in their tracks if they detect a stealthed hostile mob or player. One of the many reasons for a Hunter to learn to keep an eye on their pet.
The warlock pet Felhunter (bascially a demon dog) used to provide his allies with increased stealth detection. These days, the Voidwalker instead does something similiar while he uses his selfhealing ability Consume Shadows.
In Geist, dogs can sense you as a ghost, and know something is up when you're possessing somebody. The Big Bad knows this and exploits this, keeping dogs at various checkpoints to make sure you don't escape through a soldier. You can't escape in ghost form, due to energy shields that only let solid matter pass through.
Used in Super Robot Wars Advance. The pet dog from Zambot 3 is able to detect that something is kinda off to the protagonists, who's actually a Shadow Mirror spy. Axel gets a typical bark, but the dog barks louder to Lamia, who is actually a Robot Girl.
Koromaru in Persona 3. He's the first sign nonhumans can have a Persona (his, appropriately, is Cerberus).
A literal case in Nethack, where your pet (typically a dog) will hesitate to step on cursed items.
In the Dragon Age: Origins "Stone Prisoner" DLC, Dog will immediately sense that "Kitty" is something powerful and evil and that's before the ominous glowing eyes and ominous demonic voice come into play. The Mabari were bred by a wizard, and it's stated in the original game that the reason Marbari originally became so popular in Ferelden was because they can detect werewolves.
Works awesomely well in Fallout: New Vegas: in one of the final missions of the game, if you think to take your robo-dog Rex with you as a companion, he will detect a Legion assassin in disguise without you having to search for him. Subverted since you discover he was once a Legion-owned dog, being the companion of Caesar himself, making this arguably more of a They Have the Scent. Played straight if you have the Animal Friend perk and use a lender-dog to do the same thing.
Faction-owned guard dogs are also able to see through the player's disguises.
In The Sims 3, cats and dogs will "investigate the strange sim" if there is a vampire nearby and there will be a talk bubble on their head with a picture of bats.
When you first enter the main city in Shining the Holy Ark you see one of the villains walking around. A cute little puppy starts barking like crazy at her, a flash of light blinds you, she's gone and so is the puppy leaving a crying child.
In Long Live the Queen, you can get an important clue if your Dogs skill (under Animal Handling) is high enough. The dog barks at the scent of poisoned chocolate, saving Elodie from assassination.
In Robopon, this is inverted. The dog is evil or at least under the care of villains, and growls at Cody when he approaches.
In Everyman HYBRID, Alex's dog Sparky detects The Rake long before its first attack on Alex, and may well have been keeping The Rake from attacking while Sparky was in the room. Interesting given The Rake itself is a freakish dog-man.
Because, as we all know, no one evil has ever gotten on the good side of a dog. (cuts to picture of Adolf Hitler with his dog) Oh, right.
When Table Flip plays Betrayal at House on the Hill, they discover a Dog companion named Fidonkers shortly before the Haunt begins and the Traitor is revealed. Fidonkers then barks and growls at the traitor accordingly.
Parodied in Family Guy: Trying to score a date with his crush, Death steps into her pet store to talk to her. The animals inside raise such a commotion that they can't hear each other.
Also, Brian the dog is the only one who realizes Stewie is evil. Of course, that is more that he is the Only Sane Man on the show.
In "And Then There Were Fewer", Brian growls when James Woods shows up.
In Samurai Jack, a mouselike creature accepts food from him, but hisses and runs from his traveling companion Ikra who turns out to be Aku in disguise.
In the somewhat forgotten kids' cartoon The Magician, an Identical Stranger (thanks to plastic surgery) kidnaps and switches places with the eponymous main character, taking care to plant a behavior modification chip on his pet dog to ensure it will bark distrustfully at the real guy, and stop doing so at him, which is useful to fool the sidekick.
Radarr, Team Pet of the Storm Hawks, detects evil in two episodes that were broadcast consecutively.
In "Best Friends Forever", Big Bad Cyclonia disguises herself as an ally of the team. At first, only Radarr is able to sense that something's wrong.
In "The Black Gorge", Radarr is suspicious of a seemingly harmless animal who eventually transforms into a monstrous Gorge Sloth.
Inverse example: In Batman: The Animated Series while Harley Quinn, recently released from Arkham and out trying to start anew, her hyenas immediately start barking at the sight of Bruce Wayne. Harley for her part is clueless that she happens to be standing next to Batman.
In an episode of the Spider-Man and His Amazing Friends 80's series, Aunt May's puppy became a living lie detector, since thanks to her sense of smell she could point out who had his or her place stolen by shapeshifting villain The Chameleon.
A variation in Reboot: Enzo's dog Frisket is a pretty good judge of character, but he hates Bob, the main hero.
In the Gargoyles episode "City of Stone: Part 3", Bronx starts clawing at one of the tapestries just as Goliath and the clan are about to implement Xanatos's plan to end Demona's curse. When the gargoyles are gone, Demona comes out from a secret passage behind said tapestry with a mace, ready to smash Elisa to bits and dispose of the others.
In another episode, he takes a liking to an Irish teenage boy, but growls and barks at his female friend. Turns out that his friend is actually the Banshee in disguise, and that she was only his friend to keep him from discovering that he is the reincarnation of her warrior nemesis.
"Skull and Double Crossbones". When Jose asks Bandit if he wants to be friends, Bandit growls at him. Later we learn that Jose is The Mole, a spy for the Chief of a crew of pirates.
"Shadow of the Condor". When Bandit first meets Baron von Freulich he growls at him. Later von Freulich tries to murder Race Bannon.
In the Galaxy Rangers episode "Mindnet," the first clue to the characters that all is not right is when Icarus and Winter (a pair of cybernetically enhanced dolphins) protest that "Goose" is behaving strangely. (The first clue to the audience is that Killbane's voice actor is trying to imitate Doug Pris and isn't doing very well).
The Zillo Beast in Star Wars: The Clone Wars is able to tell that Palpatine is evil and upon escaping from its cell, rampages through Coruscant to hunt him down for that and Palpatine being the one sentencing the creature to death.
Appa and Momo are very good at detecting malicious intent in humans. Appa growls and reacts defensively towards the sandbenders (who later capture him) and Momo hisses at the pirate captain and his weird bird thingy. In fact, it's Appa licking Zuko that makes Aang wonder if he's really changed.
An adorable puppy greets Korra in Korra Alone and is able to see her hallucination self easily. That's because a spirit can detect other spirits too, and the puppy is actually the little leaf spirit she meets in the Tree of Life.
Somewhat inverted in Kim Possible, where pretty much all the villains seem to hate Rufus, but they like him if turned good (both Shego and Draken can be called as witnesses)
Grougal from Wakfu (a baby dragon) really does not like Qilby. But there's a whole lot of history around this.
In the second season of W.I.T.C.H., Big Bad Nerissa is capable of using illusions to disguise herself as any other person. However, Matt's pet dormouse Mr. Huggles always goes berserk in her presence, no matter what she looks like.
Although you really don't have to be specially evil-detecting to think that a stone-grey book with spikes jutting out of the cover and hidden in a secret locked room of a castle ruin is probably something you should leave alone.
Dogs (and cats) have been known to react in the alleged presence of the supernatural.
Or more accurately - when a dog reacts to something, some people assume this trope is in play and that the dog must be detecting some evil they can't see themselves.
Older Than Feudalism: In Zoroastrianism, one of the oldest surviving faiths in the world, the dog is considered one of the most sacred animals, for its ability to detect, and even deflect evil with a mere gaze.
Some dogs also seem to be able to sense the presence of tumors and even predict when epileptic owners are about to have seizures. In the case of tumors, this is because cancer cells smell distinctly different from a normal cell. In the case of seizures, the biochemical reactions that precede them cause a subtle shift in the person's scent, and other things like heart rate.
The book "The Gift Of Fear" says that when a dog growls at a shifty person it is actually responding to the owner's unease and trying to protect them.
Not unlikely at all, given dogs are capable of detecting their masters' emotions, even hidden ones, thanks to their senses. (they can hear a heartbeat speeding up, for one)
Dogs are also incredibly attuned to body language, since the bulk of their communication is by that method. Since dogs and people share a number of signals, they can easily pick on things we'd miss since we tend to focus on tone of voice.
Seizure response dog and diabetic alert dogs are trained to predict upcoming attacks and warn their owners. Even untrained dogs are told to be sensitive for sudden changes and attempt to protect humans from harm- barking, forcing them to lay down or even bite their hands, forcing them to drop sharp objects like knives and scissors before the seizure begins.
There is one account of a horse that did the same thing, and would answer math questions and puzzles by reading the unconscious body language of humans present and responding by tapping his hoof or pointing to an object. It was only figured out when a psychologist asked the horse a question nobody knew the answer to, and the horse couldn't figure out how to answer.
Another version says that the horse "counted" by hitting the ground with its hoof and stopped when its owner made a small nod - often without even realizing it, relieved that the horse had got the "right" answer. When the horse was put to count in a room without his owner, he kept on counting.
A specific example is the Pekingese, a dog that Chinese Emperors used to keep inside their sleeves. If the dog bit someone who had been granted an audience with the Emperor, it was a sign they were untrustworthy, which usually meant execution.
True crime author Ann Rule's dog always snarled and growled at her friend Ted Bundy and shied away whenever he tried to pet him, despite usually being friendly to everyone. Little did Ann know her friend was a serial killer spending his nights prowling college campuses for victims. Similarly, the cat belonging to the sorority house where he went on of his final rampages apparently ran away the night of his attack. In an interview with one of the surviving victims, she speculated that the cat must have sensed the impending threat.
Likewise, the neighbor of Josef Fritzl (the Austrian man who kept his daughter locked in the basement for years and fathered incestuous children with her) insists that his usually-friendly dog absolutely hated him.
Not strictly "evil" per se, but almost all animals have some form of precognition when it comes to natural disasters. So, in this case, humanity really isToo Dumb to Live! No, seriously. Ask anyone who's been through a major earthquake. Dogs, cats, horses, birds, they all freak out. And get out, if they can.
Most animals can hear the surface noises generated by the initial p-waves of an earthquake, though this sound is too high in frequency for human hearing. Chinchillas, which have hearing abilities roughly similar to humans, are just about the only animal that will not freak out shortly before an earthquake hits.
After the Boxing Day Tsunami, there were reports that elephants kept at a vacation spot in Thailand became agitated, put tourists on their backs and moved to high ground just before the wave hit, saving them.
George Clooney credits his pet pig Max with saving his life in 1994, saying the pig woke him up before an earthquake dropped a beam over the bed they were sleeping in. Since Clooney got the role in ER that made him a star months later, and he sleeps in the nude, we can say Max is also responsible Clooney didn't become famous for the wrong reason.
There is a persistent myth that if a dog or horse freaks out, and there is nothing visible to you that should be causing them to do so, one should look at what the animal is looking at. More specifically, to do so by looking between their perked ears. The myth states that you will see the spirits or ghosts that only animals normally see if you do this.
Dogs and rats have been trained to detect land mines or gas leaks by smell.
In the 19th century miners used to keep small birds for the purpose of detecting gas. From the troper's perspective, this would have been a Deconstruction of the trope since the birds were not there to be a living alarm bell, but rather to pass out or die before gas concentration could affect a human. This also gave us the phrase "canary in a coal mine."
Despite the above examples, this trope shouldn't be relied on in Real Life, as plenty of animals make a habit of reacting badly to strangers in general, or to specific types of people (all men, tall people, folks wearing hats, etc.), regardless of demeanor or intent. That said, it's probably not a good idea to ignore it altogether either.
Not quite "Evil" exactly, but any animal that can not detect (and prevent) a predator's "Evil" attempt to eat them, will not be around to breed the next generation of evil detecting animal. This applies to most dangers as well.
Particularly true of snakes, which many animals' brains are hardwired to notice even if they're camouflaged.
The patas monkey, a land-dwelling social primate from the African savanna, has three vocalizations for danger, meaning "snake", "leopard" and "eagle". It is speculated that human language once started the same way.
The "evil" aspect depends on a subjective value, but there's Lucky and Flo, two black Labradors that the MPAA praised for their ability to sniff out counterfeit CDs (and legitimate ones).
A blind man who was on the 71st floor of the North Tower of the World Trade Center released his guide dog from his harness so that the animal could escape (he feared that he would never make it down the stairs in time and wanted to give him a chance). The dog came back and pawed at him until it was finally obvious that he was not leaving without his master and proceeded to lead him down 71 flights of stairs to safety. Guide dogs receive extensive training but this still had to be a very perceptive animal to realize that this was an incredibly dangerous situation that he had to get his master out of.
Philosopher Diogenes of Sinope, who often compared himself to a dog and argued that people should aspire to learn from them, claimed that this was one of their virtues: He believed they always knew, instinctively, who their friends and enemies were.
Feral dog packs can discern when one of their number is acting oddly due to rabies infection, and quickly expel the sick animal from the group or abandon its company.
More than one unit mascot dog in World War I and World War II gained reputations for predicting artillery barrages.
The muntjac, a small deer-like animal from Southeast Asia, makes some characteristic "barkings" to alert other members of its species when they know that a tiger is near. Many animals of other species (and people are not an exception) have learned the meaning of this barking and become wary when they hear it. Keep in mind that muntjacs are quite small and have a talent for cammouflage, so they can save your life without you ever seeing them.
Similarly, in Africa most animals have learned to keep one eye on the giraffes. If the giraffes start running it's because they've seen something worth running from, like a lion. Ostriches take on a similar role in areas where they are present but giraffes are not.
Prosecutor Alice Vachss got a retired seeing eye dog (it was going blind) as a pet. Normally it would sleep in her office and ignore everyone, but if it got up while Vachss was interviewing a child abuse victim, that child always would need serious therapy.
Invoked with drug- and explosive-sniffing dogs, which are specifically trained to detect nasty materials and alert their human handlers to what they've sensed.