Fang Thpeak

A speech impediment that strikes vampires (and other fanged creatures, as well), where their spoken ss become ths or, more often, sh's. Here's the technical explanation for why it happens: False teeth tend to cause sibilants (s, soft c, like the first c in "circus", and sometimes z) to be mispronounced because the prosthetics force a change in the position of the tongue. Interdentals (th) are made by the tongue going against the upper incisors - sibilants are more likely to be mispronounced as interdentals when the incisors are altered or missing, as with fake buck teeth or missing front teeth More common with prosthetic fangs are for the sibilants to become post-alveolar fricatives (sh), caused by the tongue being forced back by the wider and longer canines. For example, listen to anything The Master says in Buffy season one. When he comments, "Nice dress", he pronounces it "Nishe dresh", not "Nithe dreth".

Fang Thpeak can be called to the audiensh'sh attention in two waysh:
 * 1) When the writersh make no effort to avoid shibilantsh, charactersh who have fangsh will alwaysh shpeak thish way, or,
 * 2) When the people writing the show, having one actor (or more) who wear a fang proshthetic, go out of their way to try to write dialogue avoiding the letter "S". An attempt of that nature will very likely end with the character having a rather odd vocabulary and pattern of word choishe. And the shibelentsh that do get through are glaring.

The same thing happens to a slightly lesser degree to people who wear upper dentures, and orthodontic retainers. Again, it's the mouthpiece interfering with the position of the tongue.

Other consistent mispronunciations include ventriloquists substituting g or v for b, and f for p -- it's nearly impossible to pronounce a b or p sound without noticeable movement of the lips.

Related to Vampire Vords and Sssssnaketalk. Though Joss Whedon uses it, not related to Buffy-Speak at all.

Comicth

 * Mr. Scarface in Batman comics had this problem, pronouncing all 'B's as 'G's. This was because he was actually a ventriloquist dummy who was the split personality of Arnold Wesker (appropriately titled the Ventriloquist), and Batman has used it to figure out his identity a few times when Scarface was trying to pretend he was somebody not made of wood. This speech impediment was not carried over to Scarface's appearances in Batman the Animated Series or The Batman, since it was deemed too awkward.
 * Many kids in the Asterix comics speak like this, including kid-ified versions of most of the main cast during the Flashback story How Obelix Fell into the Magic Potion.
 * Krurgor in Les Naufrages D Ythaq belongs to a race that can basically be described as bipedal walrusses, including speech-hindering fangs.
 * Another fang-induced lisp is that of the Cafou, a giant, talking sabertoothed cat in Les Lumieres De L Amalou.
 * In Empowered, the villain King Tyrant Lizard (a human turned intelligent dinosaur) and the hero Homunculoid talk like this; the former has a tyrannosaur's jaws and teeth, the latter has oversized lips and tongue.
 * The Beast in Fables, whenever his curse is only partially in effect -- he's got fangs but not a big enough mouth to fit them in, so they get in the way.
 * Mooch the cat in Mutts. He has a speech impediment that causes him to insert an sh into words when he "shpeaks", such as yesh (yes), shmilk (milk), and shmousie (mousie).

Film

 * Tim Burton's Planet of the Apes fell into this with a lot of the ape actors. Tim Roth had it particularly bad, and his character's name was Thade. That's rough.

Literature

 * Lampshaded in Red Dragon, particularly the book. Dolarhyde has a corrected harelip and cleft palate and avoids sibilant words, for example, always using "Mmm-hmm" instead of "Yes."
 * It's not vampires that suffer from this in Discworld; rather, it's their servants, the Igors (and Igorinas), though they can turn it off to be understood better. This varies from Igor to Igor. The hip young Igor who works for the City Watch has it intermittently (his relatives call it a thpeech impediment), while another Igor was able to shut it off for a complex explanation. Given the nature of Igorhood this could have something to do with what dental surgery a particular Igor has dabbled in.
 * Vimes believes people in drought-stricken countries would pay good money for Igor to say "sausages".
 * Offler the crocodile god of wealth does have this problem because of his tusk.
 * Gargoyles have their mouths stuck permanently open, and talk in a manner that reflects this.
 * Angua of the City Watch and other werewolves speak like this mid-way to becoming a wolf (when the half-moon is out), due to elongated fangs.
 * Betsy Taylor in Mary Janice Davidson's Undead and Unwed thpeakth like thith.
 * Vargo Hoat from A Song of Ice and Fire.

Live-Action TV

 * Buffy the Vampire Slayer and Angel. In the DVD commentary for the first season of Buffy, Josh Whedon explains that the prosthetic fangs the actors wear make particular sounds difficult to pronounce properly. The makeup artists and the actors learned a few tricks to get around this; custom prosthetics for major characters eased this problem. Whedon even invoked this trope in the first season Buffy DVD commentary by apologizing to one of his guest-stars for making him use the phrase "excruciating loser" in full vamp-face.
 * Juliette Landau wore her fangs for an extended period before taking up the role of Drusilla specifically to learn how to speak properly with them. Unfortunately, guest vamps lacked the time for this and some had more trouble than others.
 * Many of the Klingons and Ferengi on Star Trek's various series. Particularly noticable when recurring actors had to wear the makeup -- such as in the episodes of Deep Space Nine where Odo, Sisko, and O'Brien go undercover as Klingons. O'Brien even points it out. And Michael Dorn (who played Worf, a Klingon, for at least 13 years) found their plight rather amusing.
 * Armin Shimerman noted that the hardest word for him to pronounce with the Ferengi fangs was "hellhole".
 * On an aside, one of the more common Klingon expressions seen on-screen is teeth-grinding for annoyance. One imagines that at least some of that annoyance is method-acting.
 * Vir Cotto from Babylon 5 originally sported large canine teeth like all other Centauri, but they quickly disappeared because it made the actor talk like this.
 * In Chinese Paladin 3, the temporarily-vampirized hero has a huge problem with this. Not only does he struggle obviously and markedly with pronounciation, he often sprays innocent bystanders with spit in the process.
 * Not so much in the show True Blood (although it does happen) but in the parody video of the show.

Puppet Showth

 * The Muppet Movie: "It's a myth! A MYTH!"
 * Yeth?
 * Any ventriloquist dummies that are actually voiced by competent ventriloquists.

Web Comicth

 * All of the Trolls in Homestuck have fangs, but Sollux is the only one that has been stated to have a lisp.
 * Sollux is also the only one with doubled incisors (and it is only the incisors, and their counterparts in the lower jaw, that are doubled).
 * Trope-tan, in The Way of the Metagamer, lisps occasionally due to her Cute Little Fang.
 * Camille, Shannon's fragile little sister from Bloody Urban has crooked, front-facing fangs which result in a speech impediment.
 * Iacar from Wurr haf a diftinctive lifp.
 * the Jagermonsters in Girl Genius show enormous fangs when they speak or smile, some even have tusks, but for reasons somewhere between plot development and Rule of Funny, they speak with Funetik Aksent - cod-German or East European, in their case.

Wethtern Animation

 * Shelly Marsh, from South Park, speaks like this because of her braces and retainer.
 * Penny from Chalk Zone.
 * Beth from Total Drama Island had this is season one, because she had braces.
 * Phineas gets this when he dresses as a vampire for Halloween.

Real Life

 * A common result of wearing ordinary non-fang dentures, Sean Connery's distinctive speech being a notable example.
 * Anyone with a freshly pierced tongue, lip, or cheek tend to talk like this until they heal, as will anyone who's just had dental work and still has numb lips from the anesthetic.
 * Talking while wearing a dental retainer can often cause this, especially if the retainer is new and you're not used to it yet.
 * Try speaking a language that isn't your native one right after getting braces. Not pretty.
 * Legendary Notre Dame Fighting Irish coach Lou Holtsh.
 * Germans trying to speak English may thometimes do this as a form of overcompensation.
 * Listening to a recording of people speaking, which has a lot of static in it, may, for some reason, also sound like everyone has a lisp.
 * Sometimes this trope may be just linguistic, as in certain Spanish dialects. The "official" pronunciation in Spain is the Castilian one, where the soft "c" sound is pronounced "th" as in "thin" rather than "s". So "cero" (zero) would sound like "thero", and "cazador" (hunter) would sound like "cathador," and so on. Ironically, this is the least common pronunciation -- the southern half of Spain and the Spanish-speaking countries of South and Central America mostly use the "s" sound ("sero", "casador"...). This is probably because the modern Castilian pronunciation is relatively young, from around the 16th century.
 * Really? I live in Murcia (southern Spain); mostly we use "th" and not "s".
 * This originates with a Castilian king who had a lisp, and, well, you can't have a lisping king, can you? So, it became not a lisp, but the accepted way of speaking...
 * More ironically, everywhere in Spanish s's retain their "s" sound.
 * Woe unto the child whose teacher relays the misinformation that "in Spanish, the S is pronounced like TH". Yes, it happens, too frequently. Although the reactions when asking a native speaker "Buenoth diath, thu habla ingleth?" can be quite comical.
 * Although a few dialects do this, too -- it's called thetheo. I mean ceceo. Just to add to the confusion, some dialects do everything at once.