Scary Teeth

A character smiles, and it is revealed that his or her teeth are...disturbing.

This trope manifests in myriad ways, from teeth being all askew to being rotten, from being a mouthful of sharp fangs to metallic prostheses, and sometimes even there not being any teeth at all (cute in an infant, disturbing in an adult).

This trope works for a variety of reasons. One of the biggest is that trauma to the teeth is a Primal Fear for most people. Also, teeth are very much associated with youth, vigor and hygiene, so nasty teeth instantly give an image of sickness, dirtiness and malevolence. Sharp, fanged teeth are an universal identifier of a predator, too, so anyone who sees fangs or a maw full of sharp teeth will instantly feel threatened.

Can be an indicator of an evil character, although good ones aren't disqualified.

Examples of Scary Teeth include:

Anime and Manga

Arlong and Hody Jones from One Piece. Justified as they're both shark fishmen (a sawshark and a great white shark respectively).
Viral from Tengen Toppa Gurren Lagann.
Grell Sutcliff from Black Butler.
Gyro Zeppelli in Steel Ball Run is a rare good guy example. His teeth are metallic and have "Go! Go! Zeppelli!" engraved on them. Most of the time his teeth are drawn normally, but when he throws a mischievous smile, the engravings can be seen, which can be a little jarring.

Comic Books

In The Dark Knight Returns the leader of the Mutant Gang has filed his teeth to points, and uses them to rip out the throats of the mayor and his aides (off panel).
Tombstone, one of Spider-Man's recurring foes, looks ghastly enough due to his ghostly white skin and powerful build. So naturally, he's got his teeth filed into nasty fangs, which he gladly uses to bite his enemies to death with.
Being a terrifying crocodilian man, Batman rogue Killer Croc has a set of sharp fangs to compliment his scaly-looking skin. How scary his teeth are tends to depend on how monstrous the writers and artists want him to look (and act) at any given time, and at his worst they tend to look less like crocodile teeth, and more like something you'd see in the mouth of a mutant Tyrannosaurus Rex.

Film

Jaws, pictured above, from the James Bond films.
Jack has one for a moment in Fight Club, when he smiles at a fellow coworker during a meeting and his mouth is a bloody mess.
Shanghai Knights: During a chase scene, Roy runs into a demure-looking street vendor and starts putting the moves on her... then she smiles, revealing hideous rotting British Teeth. He understandably panics.
In Coneheads, the titular characters have this until they fix their teeth to blend better amongst humans: not only are their teeth shark-like, when they open their mouth wide, it shows endless rows of them.
Seth Brundle in David Cronenberg's remake of The Fly slowly loses his teeth as the transformation progresses, making him look more inhuman and threatening.
Sharpteeth in The Land Before Time
Scar, at least once, in The Lion King. The hyenas too. "Yes our teeth and ambitions are bared!"
When the Mouth of Sauron appears in the film of The Lord of the Rings, his helmet covers up every part of his face except for his mouth, which often grins with elongated, pointy teeth covered in black ichor. Eugh.
The Joker in 2016's Suicide Squad has chromed teeth.

Literature

Pennywise in IT can morph his teeth into a fanged maw.
Arno Blunt, The Dragon from Artemis Fowl and the Eternity Code. After getting his teeth shattered early on in the book, he gets a number of interesting - and somewhat disturbing - prostheses.
In Discworld, Cohen the Barbarian has dentures made out of diamonds. In the Disc, diamond teeth are associated with trolls, so anyone who sees Cohen's is understandably disturbed.
Apparently Randall Flagg in The Stand. "There were worse things than death. There were teeth." (illustrated rather well with a big lamprey-mouth sketch in the comic.)
Every time Vlad in Count and Countess is about to take a bite out of someone, he grins. The glimpse of artificially sharpened teeth serves as the victim's first and last warning.

Music Videos

One of the last mouth close-up shots of the "Weird Al" Yankovic's "Bedrock Anthem" (a parody of Red Hot Chili Peppers' "Give It Away") has dinosaur teeth. A memorable, and hilarious left-field gross-out moment.

Western Animation

Jafar from Aladdin, in his old man disguise.
The Grinch.
Ren in Ren and Stimpy. Especially in "Ren's Toothache", where he refuses to brush his teeth and they eventually rot and fall off.
In both Time Squad and Hudson Hawk, this is why Leonardo da Vinci ends up painting the Mona Lisa with a closed-mouth smile.
In Disney's Beauty and The Beast, one of the servants suggests that the Beast should be nicer to Belle. The Beast attempts to smile, but realizes he has More Teeth Than the Osmond Family.
The Simpsons: Dental Plan.
In My Little Pony Friendship Is Magic Nightmare Moon has fangs...which tend to look slightly creepy on a horse.
In Hazbin Hotel, demons who are confirmed to be cannibals (including Alastor, Rosie, and background characters in the Cannibal District of Pentagram City) have large, sharp teeth. Lucifer has them too in at least some depictions to date.