Prongs of Poseidon

Unless your story is set in Barbados, there are exactly four uses for tridents in fiction.

The first is for anyone associated with the sea, particularly if you're a fisherman or a god. This isn't contrived; tridents have their origins as fishing tools, after all, and Poseidon was portrayed with a great big trident.

The second is if you're evil. Really evil. Tridents don't go to moderately evil villains or Redshirts; if you're using a trident you better be Satan or seriously, ridiculously evil, and a major villain. They're too stylish for Mooks.

In the ancient world, tridents were used in warfare as a spear capable of disarming. They were also used in Ancient Rome in the Gladiator Games, in a parody of fishing - a fisherman (Retiarus) went after a fish (Secutor, whose helmet had a fish welded on top). So historical works set in Ancient Rome or Ancient Grome are justified in showing tridents, either in the games or in war.

Not to be confused with the much more mundane farmer's pitchfork, which unless it's being used to pitch hay is generally wielded in combination with a torch and a mob mentality.

Examples of Prongs of Poseidon include:

Tridents associated with the sea

Classical Mythology: Poseidon, of course, as well as his Roman counterpart Neptune, is always portrayed with a trident.
- His brother Hades wielded a bident.
King Triton in The Little Mermaid, unsurprisingly, as he's an obvious Expy of Poseidon.
- Or he could actually BE Triton - Poseidon's son, yannow.
  - Come to think of it, in The Little Mermaid TV series, Ariel might have said something about a "Grandpa Poseidon"...
  - In the Little Mermaid Broadway musical, Triton and Ursula are in fact the children of Poseidon. Which...is sort of weird, because it's later said that Triton and Ursula's father died. Aren't Greek gods supposed to be known for, well, not dying?
- Hades was given a bident (two-pronged trident); this is much more forgettable, as Hades tended to avoid open combat and wasn't portrayed with it very often. His helm of invisibility is much better-known.
Kiina and Turaga Nokama have these in Bionicle.
In Golden Sun: The Lost Age, the Trident of Ankhol is the only weapon capable of breaking through Poseidon's shield.
Real Life example: The Trident and Poseidon sub-launched nuclear missiles.
A trident is one of the many weapons you can wield in Nethack. True to this trope, it does extra damage to monsters in water. Additionally, if you transform yourself into a metallivore and eat a trident, you get the message, "That was pure chewing satisfaction!"
- Similarly, The trident is a common spear in the Final Fantasy games, and it sometimes deals electric damage/extra damge to aquatic creatures.
The shikai form of Shiba Kaien's Nejibana in Bleach which is a water element zanpaktou.
- Aaroniero Arruruerie also gets a hold of the exact same zanpaku-to after inheriting Kaien's memories, visage, and more by eating the Hollow that killed him before he became an Arrancar.
Teen Titans had a villain named Trident (who was secretly three people), and his weapon of choice was, yep, a Trident. Yet for some reason, none of the Heroes know what the weapons are called ("Fork Thingie" Doesn't have the same ring to it).
Drowned Wednesday (and later Arthur) wield the Third Key in Keys to the Kingdom, a trident. This has the double meaning of being the sea weapon and Wednesday is inflicted with gluttony, so it can also be used as a fork.
AndrAIa in ReBoot used a trident as a weapon after she grew up.
In the Roman gladiator games, the Retiarius was a gladiator with a fisherman theme; he wielded a weighted net, a trident, and a holdout dagger.
In The Hunger Games, Finnick Odair wins his Games fighting as, essentially, a Retiarius, with a trident he received form his sponsors and a net he wove from vines.
- Also interesting to note is that Finnick is from District 4, which is the fishing district.
In ADOM, the rune-covered trident, an artifact which conveys the ability to breathe water among other benefits, can be obtained by performing a task for the water dragon Shyssiryxius, who is found in an underwater cavern. (Also, there is the Trident of the Red Rooster, which strangely enough has no particular affiliation with either water or evil.)
In Dungeon Crawl, when given a choice of weapons, Merfolk replace the "spear" option with a trident. Also, Gladiators have the same substitution for every race (as the Retiarius above).
In Ronin Warriors, the Ronin member whose powers come from the sea, sure enough, wields a trident.
The Troll Feferi Peixes in Homestuck, who lives under the sea, carries one of these; a legendary double-trident called the ΨDON'S ENTENTE.
Warehouse 13 has the Minoan Trident (also known as Poseidon's Trident), the original Weapon of Mass Destruction, which when stabbed into the ground three times opens the fault lines below. If used over a volcano, it triggers a catastrophic eruption. If used over a supervolcano… kiss humanity goodbye. Appropriate, considering that while most people know he's the god of the seas, Poseidon was also the god of earthquakes.
The Pokémon Prinplup, who already had two "horns", grows a third horn when it evolves into Empoleon, a reference to the trident, and Poseidon in general.
Llyr the Sea Elf wields a trident in Tears to Tiara.
Neptune, King of the Mermen, and his eldest son Fukaboshi in One Piece. It also seems to be the Weapon of Choice of Neptune's Army. This include the captain of the New Fishmen Pirates Hody Jones, who is a former member.
The motto of the United States Naval Academy is Ex scientia tridens. Translated literally, it means, "From knowledge, a trident." This is totally meaningless until you realize they mean "trident" as a metaphor for "seapower."
Prince Ivar, the Knight of Water in The Mystic Knights of Tir Na Nog, used a trident as his personal weapon.
In the pirate-themed Kaizoku Sentai Gokaiger, Sixth Ranger Gokai Silver wields a Swiss Army Weapon trident/blaster called the Gokai Spear. His Humongous Mecha's main combat form has a drill on its arm that can open up into Trident Mode to deliver electrified slashes.
Aquaman is increasingly being portrayed wielding Poseidon's Trident.
Voltron from Voltron Force, when in the Blue Center configuration, gets the Titanic Trident. True to the trope, this configuration is best for underwater, and the Blue Lion's den is underwater.
Julian Solo becomes Poseidon's Soul Jar in Saint Seiya, and his armor has a pretty spiffy trident.. Also qualifies as Type 2, since Poseidon is an evil god and Athena's rival, as well as the Big Bad of a rather long story arc (two, if we count the anime).
In League of Legends, it's Fizz the Tidal Trickster's weapon of choice.
As his name suggests, Poseidon Org from The Movie of Hyakujuu Sentai Gaoranger has one.
Splash Woman of Mega Man 9 has this as her weapon, a Laser Blade version.
- Just like Empoleon from Pokémon, Wave Man's (of Mega Man 5) face is adorned in three horns fashioned after the trident.

Tridents associated with evil

Anime and Manga

While not exactly evil, Arrogant Kung Fu Guy Wufei from Gundam Wing is probably the most morally ambiguous of the five main characters & has a double-bladed Beam Trident as his Gundam's sole handheld weapon after its Mid-Season Upgrade. Notably this is the only part of any of the five main Gundams not to get massively overhauled yet again by Hajime Katoki for The Movie, which features Wufei as one of the main antagonists for much of his screentime & even has him Putting on the Reich.
Eneru of One Piece, at one point in his battle with Monkey D. Luffy, used his Shock and Awe powers to heat up his gold staff and re-shape it into a trident. Later on, when he's gone One-Winged Angel, he uses two of them and nearly kills Luffy. While Eneru is both evil and the Big Bad, his use of a trident is a bit ironic, considering his personality.
- Played straighter with Impel Down's staff. The average mooks are armed with tridents, and so does Hannyabal. Saldeath has a flute-spear and Sady-chan has a trident with a whip inside.
Bleach has Mayuri Kurotsuchi's shikai, although it has a very short shaft the more important part is that it exudes a paralyzing poison.
Rokudo Mukuro and Chrome Dokuro from Katekyo Hitman Reborn are no exceptions... They will skewer you with their trident... and also... Mukuro is an expert at Mind Rape.
Pandora from Saint Seiya: The Lost Canvas. And adorned with a snake motif.

Live-Action TV

Dagon of Mahou Sentai Magiranger is a odd combination of types 1 and 2. Not only is he a designated God of Evil, he's also based on a fishman monster

Oral Tradition, Myths and Legends

Satan, for some reason, always gets a trident if he's going to be using a weapon. He probably popularized it for evil characters.
- There's actually an explanation: when early Christians finally got in charge, the literal Demonization of the classical gods included giving Satan attributes of them; Poseidon's trident, Pan's cloven hooves and horns...

Video Games

Ganon loves to use a trident, using it in all Zelda games except The Legend of Zelda: Twilight Princess of his appearances in his monstrous form. In human form, however, he's never used it.
Nue Houjuu from the 12th Touhou game wields one, however she is a subversion.
Medievil's Zarok uses a trident instead of the more traditionally wizardly staff.

Web Comics

Evil Diva: The Big Guy's got one

Web Original

The Salvation War has The Legions of Hell armed with tridents, which they use to channel their natural bioelectricity. Demonic nobles tend to have fancier, more conductive weapons.

Tridents associated with war or combat

Anime and Manga

Shin Seijuro from Eyeshield 21 is characterized by a knight wielding a trident for his trident tackle. The kicker is that Shin is among the most righteous and skilled of the Japanese players and he bases his beliefs on pure merit and hard work, making him a subversion to both tropes.
Kanade Tachibana, from Angel Beats!, has a variation of her usual Blade Below the Shoulder, "Guard Skill - Hand Sonic version 3." She has nothing to do with the sea (Everyone Calls Her Angel), nor is she evil.
In Magical Girl Lyrical Nanoha, Dark Magical Girl Fate Testarossa gains an attack called "Trident Smasher" post-timeskip, which takes the form of three beams of plasma in the shape of a trident.
In Gundam Wing, Wu Fei's Mid-Season Upgrade Altron wields a twin beam trident. He's righteous (but misguided), so this falls more under the martial arts exception.

Comic Books

The DC Comics superhero Blue Devil uses a gadget-filled trident as a weapon, in keeping with his 'devil' theme.

Literature

In one book of the Dragonlance world, the main characters are stopped by a huge Ogre Knight armed with a large trident.

Video Games

You can find and use a trident in Drakensang 2.
Xing Cai in Dynasty Warriors wields a two-pronged fork (which was an actual weapon in ancient China and doesn't have the devil/sea associations) and a small shield.
The Javelin is one of the weapons you can get in Secret of Mana, though its number of prongs vary depending on what level you've forged it to with the Ultimate Blacksmith; it's used as a throwing weapon.
Toshiie Maeda's weapon of choice in Sengoku Basara is a large trident.
One of the possible weapons as a fighter in Maple Story is a trident with a fish stuck on the end...a Pike on a Pike, as it were.

Tridents associated with gladiatorial games

Film

Gladiator showed tridents as part of the many gladiatorial combats.

Literature

In the second Time Scout book, Wagers of Sin, Skeeter tricked his captors into outfitting him with a trident and lariat, the two weapons he was actually best with, by pretending to be bad with them.