You take the front half of a horse and the back half of a fish and that's a hippocamp(us); a merhorse, so to say. It is also known as a hydrippus, a sea horse, and a water horse, but those last two terms have other meanings too. Although not as popular as pegasi, unicorns, and centaurs, hippocamps have been around for just as long. What makes the difference is that for most of history, hippocamps had a presence in art only and never got any legends attached to them.
The oldest known depictions of hippocamps are from 4th Century BC Phoenicia and the creature was known around the Mediterranean, making its way to modern times through Classical Mythology. Horses were imagined as the creation of the sea god Poseidon, who'd either ride on them or have them pull his shell chariot over the waves. Several sources describe four-legged horses, but more often they're depicted as hippocamps. Hippocamps are a favored mount among the rest of the sea deity crowd too. Classical art regularly gives the creature wings. These may be taken literal, but also symbolical because wings may signify speed instead of actual flight. Most modern-day depictions omit the wings or turn them into fins. Different depictions may alter how much the fish and horse parts blend into each other; some may have purely equine front halves and fishlike tails, but others may give hippocamps flippers or webbed claws instead of hooves or long fins instead of manes.
The aforementioned term "water horse" is a catch-all for water spirits of equine form, whether permanent or part of the time. Most are part of North-West European folklore and none are hippocamps as defined by Classical Mythology, but they may be depicted in the form of a hippocamp.
Due to their mythological origins, hippocamps are commonly found in settings or regions associated with the Mediterranean, fantasy stand-ins thereof, Greco-Roman myth, Lords of the Ocean, Atlantis when presented as an undersea city, and mermaids in general.
"Seahorse", of course, is the name of a real-life fish, and to keep things confusing its genus name is Hippocampus. If seahorses and hippocamps show up in the same story, they're either the same sort of thing or the seahorse will be on the person end of things and the hippocamp on the animal end of things.
For the record, "hippocamp" is shortened from "hippocampus", which is a Latinified combination of "hippos" and "kampos". The former means "horse" and the latter "sea monster (in the sense of something with a long, flexible tail)". "Hydrippus" is a Latinified combination of "hydros" and "hippos". The former means "water" and the latter, as said before, "horse".
A Sub-Trope of Cool Horse and Our Mermaids Are Different. A Sister Trope to Seahorse Steed, Seahorses Are Dragons, and Dolphins, Dolphins Everywhere. Combine it with centaurs and you get ichthyocentaurs, which go on Our Centaurs Are Different. The hippocamp has no connection to any Brain Monster whatsoever.
- Cardfight!! Vanguard: The Aqua Force clan has a group called "Kelpie Riders". Despite the name, these Kelpies are essentially horses in aquatic colors and fish tails. Given the lore of the Aqua Force clan, they may or may not be artificially created.
- Mermaid Melody Pichi Pichi Pitch: Hippo's true form is a hippocamp with a near-perfect classic design. He's got wings, he's got a very long tail, and his hooves are bronze. However, his tail is serpentine and lacks the fins at the end. His mane alse reaches all the way to his tail. Hippo rarely makes use of his hippocamp form, however, favoring his human and penguin forms.
- Saint Seiya: Sea Horse Baian of Poseidon's Mariners has hippocamp armor. Contrary to the usual half-horse and half-fish look of the hippocamp, this one is a regular horse with only the tail replaced by a fish's. Additional fins are added here and there.
- Hippocampus, a sculpture by Mardi Storm, consists of three pieces: a hippocampus's torso (of rideable size), a tail midsection, and the fins. The pieces are arranged so that it looks like the hippocampus emerges from the floor as if it was the sea's surface. It debuted at Burning Man in 2002 and has been on display at various locations since.
- Magic: The Gathering: Hippocamps have appeared uncommonly in the game, either as steeds for merfolk, tritons and other aquatic characters or, more recently, as creatures in their own right. Hippocamps from Dominaria, the game's original central setting, have fairly standard horse bodies and fish tails, but Theros' possess large, showy fins and even crustacean plating on their mammalian portions.
- The first hippocamp was the steed of the Vodalian Knights card in 1994; similarly, hippocamps are used as steeds by the tritons of Theros, a plane based on Greek myth. A hippocampus made of water and possessing wings appears as the steed of the Cavalier of Gales.
- True hippocamp creatures count the Breaching Hippocamp and Wavebreak Hippocamp of Theros (2013 and 2020) and the Surge Mare card of the 2018 base set. All are typed as both Horses and Fish.
- Yu-Gi-Oh!: The Tatsunootoshigo card, which depicts a hippocamp, is a fine example of a cross-cultural mutual homage as it deals with the Western view of a seahorse as a horse and the Eastern view of a seahorse as a dragon. In Japan, the card is known as "Shiihoosu", which is the transliteration of the English word "seahorse". In all Western languages, the card is Tatsunootoshigo. "Tatsu no otoshigo" means "bastard child of a dragon" and is the Japanese word for seahorses.
- There's "Het Zwarte Zeepaardje"note in the first Nahomi album by Didier Chrispeels. In it, the titular seahorse saves the ocean god's son and is rewarded by being made one of the mounts that draw his chariot. All the other mounts are hippocamps.
- Under The Sparkling Sea, a My Little Pony: Friendship is Magic picture book released in 2013, introduces sea ponies and mermares. Sea ponies resemble striped seahorses with equine heads and two large winglike fins, while mermares are much larger than regular ponies, have mermaid-like fish tails and long manes, and are noted to be shy and very fast swimmers. Mermares of this sort are seen again in the IDW comics' "Friendship Ahoy" arc.
- The Bridge: There are three kinds of merhorse variants in the setting.
- Seaponies are the smallest and simplest, resemble oversized seahorses and are ruled by King Leo in Aquestria.
- Mermares look like a hippocampus with fins for forelimbs, but using the magical charms they wear around their necks they can take on the appearance of a terrestrial equine with a wet mane. They do this to either work above sealevel or, given that they are an all-female species, find a mate. Any offspring born from such unions will overwhelmingly be female and Mermares themselves, with any rare sons being the father's species. Most of them used to be nomadic, but those in the Eastern ocean amassed at Mako Island and named the founder of that island the first queen. Since then they have lived under an elected monarchy, with the current ruler being an old friend of Celestia and rival to Luna named Queen Maui.
- Sirens also resemble hippocampi and were born from the union of three windigoes forcing themselves upon a pair of mermare princesses. They specifically draw power from song magic by inducing certain emotions in people that listen to their singing.
- RainbowDoubleDash's Lunaverse: The three Sirens in Longest Day, Longest Night resemble ponies from the waist forward, albeit with fins at their fetlocks and longer crests instead of manes, and with mermaid-like tails from the waist back, with long, trailing fins instead of legs, and can hypnotize others with their songs. Notably, this story came out about two years before sirens with the same overall description debuted in canon. They're carnivorous, use their hypnotic songs to lure ponies into the water to drown and eat them, and live in both fresh and saltwater. Seaponies also exist and are purely marine, and sirens dislike being mistaken for them.
- My Little Pony: Equestria Girls Rainbow Rocks features the Sirens as its primary antagonists. While they remain in human shape for most of the movie, their true form is that of large, monstrous hippocamps capable of flying through the air, but their merchandise artwork depicts them as land-based ponies. They later make a cameo appearance in the main series, showing up in their monstrous merhorse forms in a flashback in "Shadow Play - Part 2".
- My Little Pony: The Movie (2017) features seaponies redesigned as more traditional hippocamps, with finlike manes, large flukes for tails, winglike fins on their backs, and forelegs tipped with fins. It's revealed that the sea ponies were originally hippogriffs, but their queen turned them into their current form with the Pearl of Transformation to escape the Storm King's attack of their home. Once he's defeated, some sea ponies choose to become hippogriffs once more while others remain in the water. When the main characters are turned into sea ponies by the Pearl, their design is somewhat different; they retain their original front halves, manes and hoof-tipped legs, while gaining a small dorsal fin sporting their cutie marks; the winged ones gain the other sea ponies' trailing fins, while the flightless ones don't.
- Fantastic Beasts and Where to Find Them: Hippocampi are among the beasts described in the book. They're native to the Mediterranean Sea, but a specimen was discovered by merpeople off the shores of Scotland in 1949 and subsequently domesticated. Hippocampi lay semi-transparent eggs, from which so-called tadfoals emerge.
- Percy Jackson and the Olympians: Hippocamps have appeared as early as The Sea of Monsters. They can emerge from waves like white seahorses do, but otherwise are fairly traditional, and are most often seen as servants and emissaries of Poseidon. The hippocamp Rainbow is a major recurring ally and the associated mount of Tyson.
- The wild sea horses in "The Sea Horses" by William Hope Hodgson are a mix of unicorns, white horses, and hippocamps. Nebby is obsessed with the wild sea horses his diver grandfather Zacchy tells about, for which the elderly man makes him a hobby-horse of wood and the tail of a bonito. Zacchy goes as far as to pretend it's a real sea horse locked between life and death while on land to indulge the boy's imagination. He also ends up telling Nebby that sea horses are the mounts of angels, unintentionally turning his grandson into a Heaven Seeker. When Nebby is brought along on the barge, severe misbehavior causes Zacchy to give him an ultimatum: either the boy apologizes to the crew or he'll return the sea horse to the seafloor. Nebby refuses, planning to use the diving gear to retrieve his sea horse himself later. Unsurprisingly, he drowns. Zacchy goes mad with grief and scours the seafloor for days to find his grandson's body until a storm rolls in and he hears Nebby singing the "Ballade of the Sea-Horses". Zacchy cuts off the air supply to reunite with his boy and ride the sea horses with him. Meanwhile, the storm pushes the lost hobby-horse up unto the waves like a true sea horse.
- A Song of Ice and Fire contains a good example of the overlap between the seahorse and the hippocamp. The coat of arms of the House Velaryon of Driftmark is a "silver seahorse on sea green" and official art switches between using a seahorse and a hippocamp.
- Chinese Mythology has the Longma, or dragon-horse. Dragons in eastern myth are associated more with water than fire. A Longma will sometimes appear from beneath a river and bestow blessings or honors to someone who has earned heaven's favors.
- Classical Mythology: Hippocampi were chiefly an artistic phenomenon in Ancient Greece and Rome; they appear in friezes and on coins, but most myths use regular if aquatic horses for the role. Exceptions include the Argonautica, which specifically refers to "two-hooved" horses of Poseidon. They often appear winged and as pullers of Poseidon's chariot.
- Chapter 22 of the Physiologus is where the term "hydrippus" comes from. The religious nature of the book casts the hydrippus as a prophet, specifically Moses, whom the other fish have to follow in order to reach God.
- Baby Bestiary Handbook: "Hippocampus Fry" is featured in the first volume, which focusses on the baby stage of various monsters. Of note is that hippocamps are portrayed as mammals that every so often need to breathe, though they can go without for very long. They hibernate in the deeps when it gets cold.
- Dungeons & Dragons: The hippocampus' debut was in the 1st Edition Monster Manual, and they've been around on and off since. They tend to be silver, blue or sea-green, and have fins instead of hooves and manes. They're intelligent Chaotic Good creatures that are usually seen as the mount of choice of tritons and sea elves.
- Exalted: Sea horses are Wyld creatures that live in the oceans of the far West. They look like normal equines while on land, but in the water their hindquarters turn into large, scaled fish tails. They are often used by the Fair Folk as steeds, and are sometimes tamed for the same purpose by Exalted or daring mortals.
- GURPS: GURPS Fantasy Bestiary classifies as "makaras" any creature with the back half of a fish and the front half of a land-dwelling animal. While the term is Indian in origin, the book notes that it can be applied to creatures such as hippocampi, which would be considered horse makaras.
- Pathfinder: Hippocampi are generally depicted as blue-skinned horses with fins instead of manes and hooves, and are popular mounts and beasts of burden for aquatic races. They're also in use by the people of Absalom, which, being a harbor city, has a special sea cavalry in the form of hippocamp-mounted Wave Riders.
- In Warhammer: Age of Sigmar, the fierce Deepmares used as mounts by the Akhelian Kings of the aquatic Idoneth Deepkin are based on hippocampi, only leaning more towards the fish half than the horse half in looks and sporting clawed forelimbs. They also have long horns resembling those of unicorns or narwhals that they can put to devastating use when they charge into combat.
- Age of Mythology: Hippocampus is an aquatic scouting unit which will spawn for free from the Dock once both the Dock and the Temple is built. They are exclusive to the Greek with Poseidon as their major god.
- Arche Age has the aquestria, a hippocamp-like creature, as a choice for aquatic mount.
- Blood Brothers by DeNA counts the Armored Hippocamp and the Chariot Hippocamp among its possible familiars.
- Culdcept: Hippocampus is a card in Culdcept Revolt. It looks like a seahorse with horse legs and its flavor text states that once it's fully grown, it'll climb onto shore and become a kelpie.
- FarmVille: Mystical Hippocamps are available as farm animals. As sea creatures, they can be housed inside the Marine Observatory. As horses, the can also be housed inside the Horse Paddock and the Horse Stable.
- Final Fantasy III: Tangies and noggles are hostile NPCs depicted in hippocamp form.
- Flight Rising: There's the hippocampus, a green and blue creature, and the clown hippocampus, an orange creature designed after the clownfish. They're familiars. The flavor text of the hippocampus — that it never forgets — is a reference to the hippocampus portion of the brain, which is important to the formation of memories.
- God of War III: Hippocamps are heavily reimagined. They're composed in part of water and have crustacean stingers protruding from their bodies. As Poseidon's Right-Hand Attack Dogs, they make for several boss fights. To an extent, the hippocamps' unusual design is as it is because the original idea was that Poseidon would have leviathans as his trusted servants. These were turned into hippocamps to stay in-line with Classical Mythology, but some of the leviathan traits remained.
- Grandia: Pirate Island features elongated aggressive fish as enemies. These are called marine gang in the Japanese version and are renamed hippocamps in the localization despite not at all looking like horses.
- In La-Mulana, hippocamps are an aquatic enemy that can attack on the surface and under water. Rather than two legs and a tail, they have four fins.
- Magi Stream: Amongs the Virtual Pets of are three types of hippocampi: kelp hippocampi (green), coral hippocampi (purple), and trench hippocampi (blue).
- Shadowverse: A hippocampus card is included in the Tempest of the Gods expansion. The main visual difference between its default form and its evolved form is that the evolved form has wings. It belongs to the dragoncraft class and the description reads that "[it] defends dark, icy waters" and that it is one of the most beautiful kinds of horses in the world.
- Shin Megami Tensei: Kelpies have been part of the franchise since the first game and have had various appearances, including normal-looking horses, unicorns, and the current horse-kelpie hybrid look. Kelpies have also been depicted as hippocamps in a few games, notably Last Bible III and Last Bible Special. The former is a heraldic interpretation, while the latter is close to the Classical creature, looping tail included. The Last Bible versions of the kelpie are preceded by the one in Shin Megami Tensei if..., which looks like the interpretation of the hippocamp from Saint Seiya: four-legged but with a finned tail. The last instance of a hippocamp-like kelpie in the series is in DemiKids, where if allied it gets the name Mermount.
- Tiny Castle: Hippocamps are among the creatures that can be raised. Unlike normal hippocamps, these have a human torso.
- Valley of Unicorns: Hippocampi are available pets. There's seven types of which four have two variants. Those four are based on the water-themed components of the Western Zodiac: Aquarius (Blue & Pink), Cancer (Red & Blue), Capricorn (Earth & Sea), and Pisces (Dawn & Dusk). There's also the Neptune Hippocampus, the Fire Forged Hippocampus (part of a Fire Forged quartet of mythical horses), and I Am Shark Hippocampus, which looks like a hippocampus dressed up as a shark.
- World of Warcraft: Both hippocamps and seahorses appear as aquatic mounts. They're similar models, the most notable difference being that the hippocamps have front legs and seahorses do not. The hippocamps aren't called "hippocamps", but are known as tidestallions and deepseekers.
- Neopets has the Peophin species. Fish tail, one set of (usually gold) front hooves, horse head with long mane, fin ears, and a golden helm with a jewel in the center.
- Elena of Avalor: Cuco is a hippocamp. He's a member of the Kingdom of the Sirenas and a bit of a puffed-up jerk.
- Mia and Me: Of the four Element Unicorns, the Unicorn of Water is a hippocamp. It's the guardian of the Birth Grotto of the unicorns, where it watches over and aids with the birth of the foals.
- My Little Pony:
- The franchise has had hippocamps among the Little Ponies in the first, third, and fourth generations. They're usually called "seaponies", and are sometimes treated as an additional pony type in addition to the core trifecta of earth ponies, unicorns and pegasi.
- The second year of Generation 1 introduced three new types of ponies to diversify the all-Earth Pony cast of the first year, those being Unicorn Ponies, Pegasus Ponies, and Sea Ponies. Sea Ponies are seahorse-based Little Ponies and for years were the mermaid equivalent of Little Ponies. Then came the eleventh year, which coincided with the release of the Spin-Off My Pretty Mermaids, and the Fancy Mermaid Ponies hit the market. These are literally mermaid versions of Little Ponies. This leaves the Sea Ponies in a nice middle position between hippocamp and seahorse, because there's also one comic page that shows an actual, Pony Friends-style seahorse. Her design is distinct from that of the Sea Ponies.
- Mermaid Ponies and mermaid versions of existing ponies were released at the end of Generation 3.5.
- Princess Gwenevere and the Jewel Riders: In "Jewel of the Sea", merfolk magic is used to turn the unicorns Cleo, Sunstar, and Moondance into hippocamps so they don't drown. At the end of the episode, this spell to give any creature aquatic form is gifted to the Jewel Riders. It is made good use of in "The Last Dance" during the challenge for the Jewel of the Sea. Said battle takes place underwater and while the ancient wizard Morgana can turn herself into a mermaid by virtue of her vast magical power, if it weren't for the merfolk magic the Jewel Riders would've drowned. Sunstar, Moondance, and Shadowsong become hippocamps this time around.
- The hippocamp is part of several coats of arms. It's the main critter of the ones representing the Macedonian Heraldic Society, Leitheim in Germany, and Donville-les-Bains in France. It's furthermore featured in the ones of Cardiff (Wales), Bridgend (Wales), Carmarthenshire (Wales), Newcastle upon Tyne (England), Medway (England), Merseyside (England), Schillingen (Germany), Almere (Netherlands), and Port Louis (Mauritius). A design with a hippocamp in it was one of the three finalists in the 1908 competition to create a coats of arms for New Zealand.
- Most of the time when hippocamps are included in bestiaries, they're grouped with fishes in general as this big bunch of unknown. Because the ocean was and is a tough place to explore, so information is hard to come by. There's often other 'camps around too, because it was believed that every sky and land creature had a water counterpart, which probably explains some of the more unusual names for real sea creatures, too.
- One of Neptune's moons is named Hippocamp.
- In 2014, the Year of the Horse, the Perth Mint produced a Horses of Lore and Legend set of three silver 1 dollar coins of which each coin depicted a duo of legendary horses. There are Sleipnir and the Unicorn, Pegasus and the Qilin, and the Hippocamp and the Kelpie.