Simply put, a character's protruding teeth are played for unconventional cuteness.
Most often, as suggested by the trope name, the character sports enlarged canine teeth. For maximum cuteness, at least one fang will be visible at all times, even when the character's mouth is closed, and especially if they have a Cat Smile. These indicate, generally, that the character is mischievous, slightly hostile, or a trickster, but not actually "bad."
Cute Little Fangs are pretty much a standard feature of Cat Girls and other characters who fall under Little Bit Beastly. Ugly Cute critters tend to have these as part of their unconventional brand of cuteness.
Usually limited to comedies. Does not normally apply to Western-style vampires (they do have fangs, but not the endearing kind).
May be related to traditional Japanese supernatural monsters such as oni, who are usually depicted with tusks or fangs meant to look rather comical instead of outright frightening. (This is explicitly the case in Urusei Yatsura, where a number of characters are Ancient Astronauts versions of oni and other supernatural creatures.)
This trope is also Truth in Television. Many baby animals, whetherof the carnivorous persuasionor the simian persuasion, sport them. In a stranger case, some people inherit a physical trait known traditionally as "Tiger Teeth", a genetic quirk that results in elongated eye teeth. This is particularly common in Japan (due to a common genetic quirk), where they're known as "yaeba" and typically considered cute. Many girls actually undergo a surgical procedure to achieve this effect. Teeth which are crowded in the mouth can result in pushed-forward eyeteeth, giving the same effect.
Viewers who are unable to appreciate the cuteness may refer to this as a "snaggletooth".
Contrast Fangs Are Evil.
Mara, the demoness from Ah! My Goddess, has these; in her case, they seem to emphasize that she's less dangerous than she looks — the series is primarily a romantic comedy, and so she would be out of place if she were a real danger. Plus they let her make a vampire joke in the TV series.
Yui of Angel Beats! has a pair of them, though only one is generally seen.
Rin has one, being the son of Satan, in Ao No Exorcist after his demonic power awakens. If he's pissed off or goes berserk, it falls under Fangs Are Evil.
Romania from Axis Powers Hetalia, which has led to fan speculation about him possibly being a vampire or at least liking to pretend that he's one. Belgium has them in some sketches too, but they're not nearly as noticeable.
Another male example comes in the form of the Medicine Peddler from Ayakashi: Samurai Horror Tales and Mononoke. They're more subtle than usual, so unless he's gritting his teeth or yelling (not something he does a lot) you barely see them. But they're there.
Hiyori Sarugaki of is often teased about her prominent little fang by fellow Vizard Shinji Hirako, and later by Ichigo, at which she promptly lampshades this by saying the fangs are one of her charms. She has a fang and buck teeth, and it makes her even funnier.
Haineko, the manifested spirit form of Rangiku Matsumoto's zanpakuto, has them.
Delmo in Cosplay Complex possesses them, even though she is a bunny girl. Apparently rodent teeth are not as cute though.
Tsuruya (that's her at the top of the page), whose Fan Nickname is LOL Fang-tan. In a noteworthy variation, the fang actually gives her a speech impedimentnote The fang causes her to mispronounce certain words; it has nothing to do with her sing-song style of speaking. Which just makes her that much cuter.
At least one illustration in the light novels gives Haruhi one.
In the Highschool of the Dead manga, Saya has a fang in just about every image, even when her mouth is closed — the artist seemed intent on displaying it at all times. In the anime it's visible only when her mouth is wide open.
Anzai Nobuyuki seems to stick these (or rather a single fang projecting over the bottom lip) on his main characters, especially when they're drawn in a more humorous fashion. It's really noticable on Ginta in MÄR. There is no logical reason given for why they have them.
Lampshaded in the Hentai manga Mozaiku x San Shimai, where a Tsundere girl's cute little fang interferes with... well, take a guess. Fortunately, no serious pain results.
Daisuke "Beast" Inuki from the yaoi manga My Beast has very pronounced fangs on his lower jaw. These usually make him look more intimidating, but in a few scenes they look very cute.
Ryôga from Ranma ˝. In fact, one reading of "Ryôga" is "Good Fang". These are apparently genetic, as seen when girl-Ranma convinced him she was his sister by wearing fake teeth with her flimsy disguise.
Moka and Kokoa from Rosario + Vampire sport Cute Little Fangs, being vampires. In Moka's case though, it's only Outer Moka who has proper Cute Little Fangs — the more mature-looking Inner Moka has longer, more menacing looking fangs.
Lime from Saber Marionette J and its sequels sometimes seems to have a Cute Little Fang, but it's hard to be sure — it's clearly a tooth, but it's far more rounded and blunt than the usual anime fang.
Saki has Yuuki sporting one these, because she's the resident Genki Girl, and Kana sporting another one of these, because she has that entire Cat Girl schtick going on.
Non-humans in human guise generally have them (Xellos, Filia, and Valgaav), but perhaps the only real humorous application of them is Lina herself, where they range from the "cute little fang" variant to worryingly noticeable◊.
Gourry got them on occasion, in the third season (TRY) of the anime.
Holo from Spice and Wolf, but hers are permanent and natural since she's a wolf spirit. In human form, anyway. In her natural form it's more a case of More Teeth than the Osmond Family — as anyone who's seen the mouth of an even completely ordinary wolf can tell.
Sugar Sugar Rune: The main characters, Chocola and Vanilla, are witches who have these.
Ichikawa in Sukisho has one of these in almost every scene for no adequately explained reason.
Kiyal, one of Kittan's three sisters in , has a fang. She's the most feral of the three. And yet in the same episode she debuts in, her own brother goes on a rant about how humans don't have fangs, only Beastmen have fangs. Wha-huh?
Nightcrawler has them in the comics, depending on the artist.
Wolverine and Sabretooth have them in some adaptations and when drawn by some artists, though describing any aspect of those two characters as "cute" is a real good way to have one's lifespan drastically shortened.
Jubilee gets them after being turned into a vampire. This turns her into a Cute Monster Girl, especialy when Sana Takeda draws her.
Rachel Morgan, from The Hollows, spends several novels dating a living vampire named Kisten. As part of their sexplay, he gives her a set of 'caps'; prosthetic vampire fangs that young vamps wear over their real fangs in order to help prevent 'accidents'. When Rachel, a human (basically) tries them on, she finds the change in roles from prey to predator very hot. As does Kisten.
Lily in the web-novel Domina. Maybe a subversion, since its mentioned that people who don't know who well think they're freaky.
The short story "Green Shadows" from Alan Burt Akers' Kregen series has the main character notice: "Her mouth was red, and a white tooth showed a tip just indenting her lower lip. Some folk find that no blemish but an added attraction in a pretty woman's face."
The Otherworld Series has Delilah D'Artigo, a werecat, who retains her fangs in human form. Her fangs are actually more prominent than those of her vampire sister, Menolly, as Menolly can retract them whereas Delilah cannot.
In Codex Alera, Kitai's canine teeth are frequently pointed out when she has her mouth open, the implication being that they are longer and sharper than a human's (but don't take away from her attractiveness). Kitai, by the way, is a Marat "barbarian".
In the Arcana Chronicles Lark, AKA Strength, has these in conjunction with her animal-based abilities. She seems self-conscious about them until Finnian, the Magician, tells her he thinks they are cool.
This is a fleeting example, but in the Angel episode "Smile Time", when puppet-Angel switches to puppet-Vampire-Angel to deliver the battle-ending set of attacks, the puppet naturally has fangs. And the fangs are definitely cute and little, but not actually little relative to the cute little mouth.
The Adipose creatures in the Doctor Who episode "Partners in Crime" have one of these. They're supposed to look like the lone milk tooth of an infant.
Some of the Centauri have these in Babylon 5. All of them, or at least all the males are supposed to have these, but due to budgetal and time constraints, as well as some actors with aversion towards them, they are only present some of the time.
The Cat from Red Dwarf has large, somewhat comical-looking fangs. They're the only visible sign that he's not really human. Considering the fact he's a flighty, ridiculous cat-man, they probably qualify as cute.
In the former, Sora gains them as a result of wearing vampire-like clothing. The clothes themselves look adorable in the first game, but badass in Kingdom Hearts II, though the fangs remain small and cute.
Female orcs and trolls always have small but visible tusks, and have the option of being rather pretty. Although draenei also have decidedly sharp teeth — and the females, at least, are attractive — said teeth aren't visible when a character has their mouth closed.
Night elves have fangs too, which is one of the reasons some followers of Warcraft lore think they're descended from the trolls (the fangs being residual tusks). Like with the draenei, though, you can't see them when their mouths are closed, which is most of the time. (Blood elves appear to have lost their fangs.)
Tauren, too, have sharp fangs that are only visible when they open their mouths. It may seem strange seeing as cows are herbivores, but the lore made it pretty clear that tauren aren't.
Marta consistently has one in the left side of her mouth for some inexplicable reason. Marta's fang is called out by Ratatosk when Marta finishes a battle with her "Radiant Roar" Mystic Arte. Emil later calls it out after battles when Ratatosk is sealed away (again) temporarily, albeit more timidly.
Marta: Beautiful roses have thorns! Ratatosk: Beauty, Marta, has a fang... Marta: Be a little more enthusiastic!
Evil sadist bitch Alice also sports a pair occasionally, but only when she's making a particularly evil face, so it may just be there for effect.
Remilia Scarlet is usually depicted with these. Fitting, as she's a vampire. The "cute" aspect is often played up in fanworks, with her maid/servant Sakuya often found daydreaming of that fang and what it can do.
Remilia's younger sister and fellow vampire, Flandre, is occasionally shown with these as well (mostly in fan-works). Of course, given Flandre's psychosis and prospensity for destruction on a massive scale, sometimes the fangs are distinctly neither little nor cute.
On a lesser note, Rumia, whose fang is used to express her childishness (and tendency to be one of the few characters that is actually mentioned to eat humans).
Ace Attorney: Furio Tigre and Shi-Long Lang, both characters who have strong ties to predator animals in their names and appearance, have pointed incisors.
Satoko Hojo from Higurashi no Naku Koro ni. However, her fang is hardly noticeable in the first season of the anime, and even in later seasons it's often not seen. It's seen in many of her sound novel poses, both the remakes◊ and originals.
Umineko no Naku Koro ni has tiny fangs all over the place. Battler, Jessica, Maria and Beatrice (among others) have them, though only Beatrice and Maria keep theirs in the anime and PS3 port.
Makoto of Kanon, an early clue to her fox origins.
The green-skinned cleric Benn'Joon aka "Benny" in the webcomic Looking for Group. She often has one tusk-like Cute Little Fang peeking out from her lower jaw over her upper lip, especially when she's pouting.
In Pandect, Ice, a Dog Ace (a dog who has a soul and can also appear in human form), has slightly longer canine teeth than most humans. Of course.
Most trolls have pointy teeth, but Nepeta is really the only one who manages to make them look utterly adorable.
Tavros, at one point, is shown curled up against the wall, sleeping, with his fangs protruding from his smile. Since Tavros is incredibly timid and the teeth of trolls in his social caste are supposed to be blunt (after this is established, the other lowbloods, especially Karkat, are drawn with rounded teeth(no he isn't)), the only explanation is that it was to make him cuter.
Secret from Keychain of Creation has a pair - they're normally hidden, but when she's stressed or using her Abyssal powers, they emerge.
At the Super Hero SchoolWhateley Academy there are several: Paige (a were-catgirl), Carmilla (mostly demon), Tennyo (too complicated to explain here). Tennyo's been told several times that her fangs are 'cute'.
Beast Boy of Teen Titans, has one among his lower teeth, protruding from his mouth at all times to add to his unconventional cuteness and underline his animal status. Curiously, he tends to carry it over in all of his forms — even when they don't even have teeth. His comic book version initially lacked this, but eventually he was redesigned (though at least they had the decency to do so in-story) with the fang intact due to the show.
Mouse from ReBoot. The fact that she has fangs is mentioned by other characters but never explained. In the end, they do indeed appear to be there solely to make her seem more dangerous.
Known as "yaeba", "double tooth", in Japan; where it is fairly common, and typically considered cute and attractive. Due to crowding resulting from small, narrow jawbones, one or more canine teeth are pushed forward of the rest of the teeth. Many believe it to be the origin of the "Catgirl" look. See the Yaeba Superstar page for more examples. As western-style dentistry becomes more popular, straightening teeth and removing yaeba is also becoming more common.
Some Japanese girls actually pay to have Yaeba artificially installed.
Male musk deer possess surprisingly large fangs, with which they fight each other for mates and defend themselves from predators. Being tiny deer, this is automatically infinitely cuter than it has any right to be.