Tipped Off by the Tail

Eh, I'd still hit it.

Luigi: Hey, I don't wanna be rude, but wasn't that a tail I just saw?
Kootie Pie (as Lady Fettuccini Alfredo): A tail? Me?! Why, I have never been so insulted in all my life! Here I am, locked in these dungeons for days without a thing to eat, and now you're accusing me of having a tail?!

— The Adventures of Super Mario Bros. 3, "The Beauty of Kootie"

Is that a hot babe walking down the street? Damn, is she fine! That sexy lipstick, those voluptuous curves, that tail over her smokin' — wait a minute...

Sometimes the mere reveal of the tail exposes an animal/monster/alien in a human disguise or draws attention to a change the owner was blissfully unaware of.

While this may not necessarily be a major inconvenience, especially if it's a setting where tails are the norm, it may still be problematic if they need to hide their animal identity from the animal police.

A common form of Glamour Failure and Morphic Resonance.

Compare with Tattle Tail, where someone's tail reveals emotions that they would prefer to keep concealed. Not to be confused with Incredibly Obvious Tail.

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Examples:

Advertising

• One of the Jay Bush and Duke ads has Duke the dog trying to con Jay into letting him sell the secret family recipe for Bush's Baked Beans by posing as a Bedsheet Ghost. However, Jay isn't fooled and when Duke asks how he knew, he points out that "Grandpa didn't have a tail."

Anime & Manga

• Pokémon: Zoroark: Master of Illusions: Zorua has the power to disguise himself using illusions, but it's not perfect since he's only a baby. When he is talking to Ash and his friends while turned into a copy of Brock, a reporter named Karl overhears their conversation. To keep Zorua hidden, Brock tries to pretend that Zorua is his twin brother, but Karl notices Zorua's tail.
• Inuyasha: When Kagome and Shippo are captured by Koga and his wolf clan, Shippo disguises himself as Koga, and attempts to escape while telling them he'll escort their prisoner. As they walk towards the cave's exit, a wolf notices that "Koga's" tail looks different and bites it, causing Shippo to yell out in pain and change back.
• Seton Academy: In the zebra-girl Chroe Mashima's debut, she parades herself around as the head of the horse herd claiming zebras are rare, striped horses. When Jin exposes that zebras are actually more closely related to donkeys, Chroe panics and accuses him of lying. When asked to provide proof, Jin indifferently lifts up Chroe's skirt and pulls out her tail, showing that it is only fluffy at the tip, just like a donkey. Chroe flees in embarrassment, ironically more upset over people seeing her tail rather than her underwear.

Comic Books

• Art Spiegelman's Maus illustrates his parents' experience in the Holocaust by depicting Jews as mice and Germans as cats. His father Vladek testifies that he and his wife Anja tried to pass themselves off as Poles, but Art's mother was more identifiable as Jewish by her appearance. The comic depicts this as the couple wearing pig masks, but Anja's mouse tail sticks out from beneath her cloak.

Films — Animation

• Scooby-Doo! and the Reluctant Werewolf: After Shaggy transforms into a werewolf, he and Scooby Doo try to sneak away from a monster-hunting mob by hiding Shaggy's face with a popcorn box. Unfortunately, Shaggy's tail blows his cover.

Films — Live-Action

• The Shaggy Dog (2006): Near the end of the film, Dave accuses Kozak of illegal animal experimentation in court; both being victims of the shaggy dog curse although Kozak is unaware. The court is initially dismissive of Dave despite Kozak's dog-like behavior. But Dave tricks Kozak into triggering a partial transformation which causes him to sprout a tail, incriminating him on the spot.
  Dave: [pointing at Kozak's tail] If that isn't proof of animal experimentation, I don't know what is!
• Spaceballs: Subverted. When Barf (a half-man, half-dog alien) and Lone Starr are disguised as prison guards, Barf's huge tail sticks out of the back of his uniform. Another guard catches sight of Barf's tail as he disappears round a corner, but in a comedic twist just shrugs it off muttering "Nah".
• The Wind in the Willows (1996): Mr Toad’s friends Badger and Rat try to sneak into his mansion (which had been taken over by the weasels) to help him by disguising themselves as weasel guards. Although one weasel is initially fooled, Mr Rat’s very non-weasel-like tail gives them away.

Literature

• Redwall: Inverted in Mossflower. Mask, an otter who is a Master of Disguise, is caught when it turns out he doesn’t have a tail. The jealous weasel captain Cludd stomps on the tail of Mask’s fox disguise, but it comes off, revealing he isn’t a fox.

Live-Action TV

• That's So Raven: Narrowly avoided in "Don't Have a Cow". Raven and Chelsea start to turn into cows thanks to an accidental magic spell. When they try to talk to Eddie while hiding their new appendages, Raven's tail starts wagging, but they manage to hide it before he notices.

Mythology & Religion

• Possibly one of the earliest examples of this trope, in Japanese mythology, kitsunes can disguise themselves as humans, but sometimes their disguises will be discovered by a tail poking through their kimono.
• Exploited in the 16th-century Chinese story the Merchant's Son, when the protagonist, a plucky boy, spies on the fox-demons in his village by wearing a fox tail (off an actual fox), so they assume he's a fox-child who hasn't yet figured out how to disguise it yet and talk freely around him.
• Some versions of the Scandinavian forest-beings called hulder appear as beautiful women with cow or fox tails, which they keep hidden by always turning to face you. Other versions have a hollowed-out back like the gap in a rotten log, and take similar pains to conceal it.
• Some of the stories around the Monkey King Sun Wukong involve how he had a very difficult time mastering the 72 Earthly Transformations, as he constantly struggled to figure out how to hide his tail. A few stories even suggest that he never truly mastered the technique, and that you should check for a tail when interacting with a suspicious person.
• Some werewolf folklore inverts this trope; if a wolf is missing its tail, that might not really be a wolf, but a human transformed into one.

Puppet Shows

• Fraggle Rock: In one episode, Boober scares Wembley to test his baloobius (the fluffy end of a Fraggle's tail, which flares when a Fraggle is scared). Trying to get back at him, Wembley hides under a pile of laundry, planning to jump out of it to scare Boober—who isn't fooled, as he sees Wembley's tail sticking out. Boober hides, waits for Wembley to jump out of the laundry, then scares him again.

Video Games

• Ghostwire: Tokyo: A pack of Tanuki were visiting the city just before all the madness started and got scattered in the chaos. You get a side mission to find all of the lost Tanuki hiding int the city, which involves looking for regular inanimate objects with fluffy racoon tails, as that's the only part they can't disguise.
• The Elder Scrolls V: Skyrim: While in the Thalmor embassy, you have the chance to sneak around and disguise yourself as a Thalmor agent, with different levels of possible success depending on your player character's race, with High Elves having the most freedom, and other elves or humans just having to be careful. However, a Khajiit or Argonian will have no chance of success; any Thalmor agent who sees one of them trying to disguise themselves will say "Is that a tail?!" and attack.
• Neeko, the Curious Chameleon of League of Legends is able to shapeshift to mimic people around her, an ability that Depending on the Writer waffles between "flawless" and "almost flawless", usually based on the Rule of Funny. In the latter cases, she still retains her giant lizard tail and floral hair decorations.

Western Animation

• Family Guy: In "The Blind Side", Brian dates a blind woman who doesn't like dogs, and when he's invited to have dinner with her parents (who can see), he disguises himself by wrapping himself in bandages and claiming he was in an accident. When Brian's tail starts wagging through the disguise, Stewie (whom he brought along as his "nurse") tries to hide it by chopping it off and cauterizing the stump with a candle.
• Looney Tunes: Bugs Bunny's disguises are often compromised by his fluffy tail poking out.
• My Life as a Teenage Robot: In "Daydream Believer", Jenny's new dream chip malfunctions and starts seeing everyone as monsters, with Tuck appearing to her as "Goatboy Junior". In one scene, Tuck "disguises" himself by wearing a costume of junk, which Jenny sees as the normal Tuck, but when a lid covering his rear falls off, she sees a little goat tail and gets wise to the ruse.
• The Smurfs: Inverted example. Hogatha disguises herself as a Smurf in "The Fake Smurf" but lacks their distinct little tail, so she has to use a fake one. It eventually falls off while she's in the Smurfs' village, which exposes the fact that one Smurf is an imposter once the fake tail is discovered. Immediately afterward, when the Smurfs are checking each other's tails, Hefty demands to see Hogatha's. She flees, trying to hide among other Smurfs, but Hefty immediately notices the lack of a tail.
  Hefty: Hey, stop! It's the fake Smurf!
• Tiny Toon Adventures: In "The Looney Beginning", Buster and Babs try to get the stolen scripts back from Montana Max by disguising themselves as Elmer Fudd and Yosemite Sam. Of course, in the spirit of Bugs Bunny, their tails give them away.
• Happens on a regular basis to Scooby-Doo, when he and Shaggy dress up in costumes or pose as statues/mannequins to fool the latest creep-in-a-suit.

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