Mermaids, or more generally merfolk, crop up quite a lot in fiction. However, like most mythical creatures and monsters, they are a little different every time and have different rules applied to them. Their dispositions, morality, and alliances vary depending on the author, and whether or not they can become human is a question everyone answers differently. Even their general appearance isn't fixed: see the picture.
Merfolk are generally portrayed as beautiful women (mermaids) or handsome men (mermen) with fish- or dolphin-like tails in place of legs. Sometimes they are not so pretty, but still fall under the Cute Monster Girl heading. Others are more blatantly sea-creatures with few human characteristics and are quite ugly, for example the Harry Potter merman (mermaid?) pictured. Sometimes they have features reminiscent of other, more exotic seas-creatures, and sometimes they are sea-animals that become human-like under certain circumstances.
Some joke that merfolk have the fish half on top instead of on the bottom in order to resolve "the Mermaid Problem". Mermaids who are more human-looking (and modest) tend to wear Seashell Bras. And then there's underwater folk like Aquaman, Namor the Submariner, The Man From Atlantis etc. who look like normal humans for the most part but can survive and breathe underwater and may have some odd physical adaptations that are well-hidden until they return to the sea. It should also be noted that most mermaids have the "fish half" in a cetacean (horizontal fluke) configuration, rather than a fish's vertical configuration, since this is closer to how an actual human swims, though exceptions exist. Of course, cetaceans are terrestrial mammals which have evolved to an aquatic lifestyle, so this is probably fair enough.
Merperson to Human:
Sometimes merpeople can turn into humans, or vice versa, under certain circumstances. The way this is done and differences in the "rules" surrounding this are numerous:
The Splash Method: The mermaid can become human when they are dry and turn back when wet. Sometimes this is something all merpeople can do, and sometimes there are limits on how long they can do it.
The Fredericka Bimm Method: Mermaids can switch forms more or less at will. Unlike the Splash method, getting doused with water is not a problem.
The Magical Item Method: Common in some mythologies but, curiously, not often utilized in modern fiction. The Mermaid has a magical item which allows her to change from one form to another. Some of these items are obvious; a Selkie needs her shed seal-skin to return to her home in the sea. Or a mermaid could pass for human on land in a wheelchair. Others are... kind of random; a Merrow needs a hat made of red feathers to (depending on which variant on the legend you are reading) return to the sea or assume human form.
And of course, some mermaids don't posess shapeshifting abilities at all — no matter how much they may want to change, the half-fish appearance is their default, permanent form.
Good or Evil?:
Sometimes mermaids are portrayed as evil seductresses (sometimes called devil fish) who lure sailors to their deathsnote this is based on the myths of sirens which were originally birdwomen but who are generally portrayed as mermaids these days due to language confusion. In most mythologies, they display the inscrutable and sometimes dangerous amorality of The Fair Folk, especially if mermaids are treated as marine fae. Sometimes they are good but flirty and mischievous. Sometimes they are mostly innocent and naive. Sometimes they are honorable, and of these, several resent us for treating their ocean home so badly for so long. Sometimes (just sometimes) they are just like us and have all sorts of personas. Very often, though not always, they are Tsundere, or even outright Yandere types. Octopus or Squid-based mermaids—sometimes called cilophytes or cecaelia—are usually portrayed as evil, probably owing to the influence of Disney's sea-witch Ursula, or perhaps because of the "alien" nature of tentacles.
One thing that also tends to vary among depictions of mermaids is their ability to survive on land: some depictions show them being completely unable to breathe out of water, others show them being able to stay above the surface but needs to be wet to avoid drying out, and some show no qualms at all about leaving the sea and can stay out of water seemingly indefinitely (except for not having legs).
Most merfolk can communicate with fish and sea creatures, regardless of whether they eat them or not. Expect merfolk to remain naked no matter how deep they swim or how little protection their skin has, though their bodies might have some unseen protection against the cold and high pressure of the depths.
Unscaled Merfolk is a Sub-Trope, where the non-human parts are not a scaled fish; see also Selkies and Wereseals.
See also Mermaid Problem, Shape Shifting Lover, The Fair Folk, Sirens Are Mermaids and Friendly, Playful Dolphin, Heroic Dolphin and Sapient Cetacean.
May occasionally overlap with Fish People, though they are generally more of an anthropomorphic fusion than a half-and-half mix.
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Anime and Manga
Blue Submarine No. 6 has a genetically engineered, evidently entirely female, race of mermaids. Typically they pilot underwater lobster-ish mecha as the front line troops of the antagonist forces. While fitting the Cute Monster Girl trope in many ways, they are not your traditional style of mermaid, considering that they have a tendency to bite.
Sophia and The Mermaid Tribe and The Merman Tribe in Doraemon.
In a story arc of a Mazinger Z manga, a race of giant Fish People from another dimension called Chip Kamoy attempted to invade Earth. In order to communicate with human beings, they kidnapped a human girl and transformed her into a mermaid: her legs were replaced by a long fish tail, and fins grew from her head. She was definitely good, though, and tried to help The Hero Kouji Kabuto. Unfortunately her "masters" executed her in punishment.
Usamaru Furuya's Short Cuts has a mock poll about mermaids, asking which would you prefer, regular (fish bottom) or reverse (fish top) mermaid. The final "moral" is that people liking regular mermaids is the reason the population is decreasing: Put up with your partner, even if they have a fish face.
Mermaid Melody Pichi Pichi Pitch involves mermaids who follow "The Splash Method"; though it also has elements of "The Little Mermaid" in there. If a mermaid tells a human her secret she'll turn into sea foam (supposedly; it's never tested, and the manga implies it might be a lie spread to keep up the Masquerade). Additionally, her human form is quite different looking from her mermaid form besides just the obvious lack of tail. The main character also sacrifices her voice - but just her singing voice (she can sing beautifully as a mermaid, not so much as a human). On top of this, they are also Magical Girls with Magic Music.
A one-shot manga tale by Rumiko Takahashi had a passionate romance between a human boy and a mermaid - which ended as soon as they kissed. (Mermaid breath tasted like rotten fish, and human breath tasted like rotting plants. Yuck both ways.)
The mermaids from Takahashi's Mermaid Saga are vastly different than most, as they are monstrous, flesh-eating, and very hard to kill. Eating their flesh has a Million to One Chance (really) of granting immortality — and otherwise kills you,if you're lucky.
My Bride Is a Mermaid's mermaids demonstrate the Splash method listed above. They can remain in human form as long as they don't get sufficiently wet (which isn't too much.) Nagasumi spends a lot of time drying off Sun just to avoid having her secret discovered by others. Adults grow out of this, and it is possible for younger mermaids to resist the change with limited success if they are disciplined enough. Their songs can cause a variety of status effects. Otherwise nice people, aside from being Yakuza... Also, one of them is the Terminator who can take a Kill Sat to the face without flinching. Yeah... Nagasumi gets in some serious trouble with him due to a severe misunderstanding while drying off his daughter.
Rave Master has Celia, a very beautiful mermaid who naturaly falls in love with Haru, much to Elie's displeasure. Her magic is strong but useless in the absence of water. She is the younger sister of the Queen of the mermaids, which of course makes her a mermaid princess.
Husky is a Bishounen Merman from the manga +Anima. Like most +anima, he's the only one of his kind (the only one who is fish like, that is). His legs turn into a fish tail at will. If he is about to drown, he'll change in order to be able to breathe: this is how he was forced to do it in the circus. It apparently takes him some effort to turn into his human form. He's also perfectly amphibious. And he's probably the prettiest mer-man around. When he's introduced, he's performing in a circus as the Mermaid Princess.
In the Slayers anime series, there's a mermaid that's a fish. Literally, just a fish... with tetrapod arms and legs. Not a reverse mermaid. A * whole fish* with legs, arms... oh, and lipstick. There are other merfolk seen in the earlier (1st and 3rd) series; turns out they all look like fish with arms and legs.
Their most amusing (and oddly touching) appearance is in a Slayers Try episode: a young fishgirl and the shipwrecked sailor she saved are in love with each other, and use a spell that will allow a fishperson to be with a human if they truly love one another. The spell actually works, and she changes into a beautiful human girl... and he turns into a fishman!
A similar type of Fish Person is found in Papuwa, examplfied by Tanno, the cross-dressing fish identified as such by his lipstick and his feminine legs clad appropriately in fishnet stockings.
In One Piece, Mer-people come in two flavors: Merfolk and Fishmen. Fishmen are Fish People that are ten times stronger than humans on land and twenty times under water. Merfolks are The Little Mermaid type. At the age of 30, the tails of mermaids (but not mermen) split and may be used as legs. Even so, they are the fastest swimmers in the world. In both cases, the types of fish they're similar to, and thus their physical features, greatly vary. Merfolk and Fishmen can intermarry, but instead of being hybrids each child can be either a mermaid/man or a fish(wo)man.
In the short horror manga School Mermaid, the mermaids actually look like schoolgirls in bathing suits, with an ever-present creepy smile on their face, the ability to swim through solid matter like it was water, and at top speed at that, only come out at night, and can only make screeching sounds. Oh, and if a human girl who's hunting them is unable to kill and eat a mermaid by sunrise, they'll all rise up, drag her down into the floor, and turn her into a mermaid herself.
In 07-Ghost noel mermaids have furry rings around their neck and waist, and the ability to change the appearance of their face at will. They also can only express themselves in melody.
The sea people from Umi Monogatari resemble humans exactly (not counting Pointy Ears, which nobody seems to notice) except for the need to breathe underwater, though they also have magic rings that can solve that problem. They also tend toward skimpier clothing than most humans, something that is noticed when they go on land.
Digimon Frontier: One of the antagonists, Ranamon, is a human hybrid fairy digimon with control over water. Despite not having the typical mermaid traits (human upper body and fish tail), she definitely constitutes being a mermaid. She is a big fan among many digimon due to her attractive appearance, but this changes when she digivolves into her hideous, tentacled beast form, Calmaramon. Her benevolent mega form, Ancientmermaimon, is more typical of a mermaid.
Level E has mermaids with the rather unusual power to detect any attempt to lie to them... by their tongue involuntarily shooting out and stabbing the liar to death. Unfortunately, it doesn't stop them from being sold into sex slavery and pretty much wiped out, because a loophole is found to evade the ability but the mermaids don't realize it.
Isma from Berserk is introduced as a lonely genki fisher girl whose lack of nudity taboos borders on Innocent Fanservice Girl category, unlike a typical Mermaid Isma appears unaware of the fact that she isn't human and claims that she isn't even sure that Mermaids exist, readers though get fairly quickly a subtle hint that suggest something else in an ominous style true to this seriesLater it turns out that she didn't lie about being unaware of what she is, what's more her cheerful and kind behaviour really is her true personality which is quite a surprise considering thesetting.
Hekikai No Aion: The mermaids need to consume "psyche", only obtained by killing people with tsunamis or earthquakes. In-universe, they're responsible for the sinking of Atlantis, Mu and the destruction of Pompeii, and they are the primary targets of the protagonist. The youngest of them is able to change into a fully human form to move around on land.
Zekkyou Gakkyuu features a story about mermaids living in a school pool who take only the beautiful to their land to make them even moreso. The mermaid is real, but it's actually a linked chain of animate drowned corpses that kills girls and adds their bodies to itself.
In Vampire Princess Miyu, Rima Minami sees to be the typical mermaid with fish tail and able to breathe underwater kept into a fish tank by her father. The twist is that said dad is a Shinma (and her mom is said to have been a normal mermaid), and Rima also has a twin sister named Mari who does not have anything mermaid-ish, but shares a Psychic Link with Rima that lets the captive mermaid-Shinma girl see the outside world.
Casual Inevitability Contactset around Hoi, Vietnam are depicted as dark/black-skinned that either come in black and blonde or the latter grow into the former along with horns,Overly-Long Tongue, Facial Markings, Cute Little Fangs, more tattered looking tails than usual and would be almost otherwise a bit more Cute Monster Girl than Ariel except they have webbed fins/pseudo-legs like the Soul Collectors from Inuyasha that they can somewhat use like tentacles on land since they don't transform. The shota and his uncle in the story "fish" for them with baitless hooks that's more like a different kind of"hooking up" where the mermaids use the lines like rope to be harmlessly reeled on-board to decide if they want to have consensual sex with the fisherman who caught them. The boy lucks out with a full-grown Shota Con with noSeashell Bra that gives him the power to breathe underwater via French-kiss while a Token Mini-Moe has a crush on the uncle since she gets herself caught by him twice and wants to stay with him afterwards.
Magic: The Gathering has, on-and-off throughout the years, had a variety of merfolk cards. They range in appearance from deep-sea fish to Abe Sapien. They tend to be small utility creatures, and have been a standard Blue tribe. Interesting enough, an article on the official site explained the merfolk are magicial in nature, and gave some basic bio specs. Granted, none of the stuff really comes up in the on-card fluff. There are a few subraces of particular note:
The Rootwater merfolk from the Rath Cycle are hideous, fish-faced monsters that kill members of any other race without hesitation. They apparently started out as normal humanlike merfolk but were altered via genetic engineering.
The merfolk of Saprazzo in the Mercadian Masques set have the power to switch between legs and fins. On land, they just look like lanky, hairless, blue-skinned humans, sometimes with head fins. They have a vaguely Middle Eastern culture.
The merrows of the Lorwyn block are the usual attractive human-looking merfolk, though they have brightly colored skin and flamboyant fins; they're merchants and porters with good diplomatic skills. When Lorwyn is transformed into the dark world of Shadowmoor, though, the merrows become black-hearted monsters with much more piscine features (very similar to the Rootwater merfolk of Rath) who prey on other races as pirates.
The comparison here: Lorwyn, a bit fishy but friendly-looking. Shadowmoor, looking like a cross between a catfish and Cthulhu.
Interestingly, because of their position as small blue creatures, combined with the fact that blue gets small flying creatures, there are a number of merfolk in Magic that can fly. The weirdest example would have to be Gaea's Skyfolk, a "Elf Merfolk" who flies. There has been no attempt to explain this, unlike the other 8 merfolk flyers.
Currently, MtG merfolk come with legs, which allows there to be sea themed races without the entire oddness of creatures that can swim in water, and so should beat any non-water breather there, but that can't do much on the atmosphere.
Those would be Zendikar's. They are also heavy on flyers, some - but not all - due to riding large birds or flying manta rays. They also had a rather well fleshed out tripartite religion centering on Leviathans and Angels which unfortunately turned out to be based on distant memories of the Eldrazi. They also have continuing extraplanar representation in the form of the Planeswalker Kiora.
The Odyssey and Onslaught blocks introduced Cephalids, a squid-based type of merfolk that were more squid than humanoid, with soft bodies and limited mobility out of water. In keeping with the "evil tentacles" motif, most of the cephalids were right bastards.
Superman's ex, Lori Lemaris, is a mermaid (it's... complicated) who lived on land by using a wheelchair and keeping her tail covered. Eventually she was magically given the power to turn into a human (any contact with water changes her back, though.)
In Teen Titans Go!, the comic adaptation of the AnimatedAdapatation of Teen Titans, Gill Girl is a Fish Person who, unlike most, isn't human-looking enough to be your typical hot mermaid. She is tearing up the city searching for her mate... revealed to be a normal, non-anthropomorphic turtle. Other than her (and we don't get her story) most Atlanteans are of the Aquaman/Aqualad entirely human-looking style (except for the solid-black eyes, in Aqualad's case [purple-irised in the original comics].)
Gill Girl was possibly inspired by The DCU character Lagoon Boy, who was introduced in Erik Larsen's Aquaman run as a representative of one of various oceanic humanoid races who comprised Atlantis' minority groups (merfolk and Aquaman-type humanoids being the majority). L.B. is basically a teenage version of the Creature from the Black Lagoon, with the ability to expand like a puffer fish.
Marvel Comics Atlanteans, and various subraces, are an interesting example. Their appearance generally depends on what continuity is active at the time. In the Silver Age, Marvel Atlanteans were the human inhabitants of Atlantis until it sank, whereupon they were turned into blue-skinned water breathers by the god Neptune. In earlier comics, the men looked like fishmen and they weren't actually from Atlantis. When Namor was reintroduced in the 1960s with the Fantastic Four, Stan Lee and Jack Kirby retconned the Submariners (what they used to call Namor's mother's race) to Atlanteans as water breathing blue skinned people. Hybrids apparently can breathe air, are stronger than either race, and can fly with wings on their feet. There were also green-skined fish-men who were the later Lemurian offshoot, mutated by the snake-god they worshiped.
Namor is the only one with the wings though, and that's because in addition to being a Half-Human Hybrid, he is also a mutant.
CrossGen's Scion has a variety of humanoid sea-dwellers who were descended from a genetically-engineered servant species, but had broken free and lived in an Underwater City. They were smallish, gray humanoids with two legs and gills.
And their leaders (?) look like giant, luminescent brine shrimp. No, I'm not joking.
In Gold Digger, Atlanteans are humanoid amphibious aliens with dolphin-like skin, gills, and small fins on their arms and legs to assist in swimming.
Meat Cake has Effluvia, a beautiful but slutty mermaid who flirts with sailors — and then lures them to their deaths and turns their souls into jewelery. She travels about on land with a wheelchair, usually.
In Hellboy, Abe Sapien is basically a gill-man. A good-natured, erudite, streamlined gill-man.
The ElfQuest spin-off Wave Dancers played with this in several ways. (The "mermaids" were space elves modified by magic to live in the water. Exactly how they were modified varied wildly.)
One of the females is the octopus variation, but not shunned or evil. Her lower half looks like a hoop skirt made of tentacles.
Fathom is about a race of beings who can turn into water or use it as a weapon, among other things.
In Keepers of the Elements, there are the mermaids who live in Aequori Kingdom. They are all shown to wear regular tops on their top halves. The previous Water Keeper Marissa lived there for some time and even fell in love with one of mermen there, eventually getting him changed to a human somehow and going to Earth to live with her where they eventually had a family together.
In An Anthem For Sheltered Bays, at first Eren and his village appears to be the standard half-human, half-fish mermaid and Mikasa is an octopus based mermaid but the former are escaped genetic experiments that were originally human and the latter were a branch that evolved within the sea. Mikasa's people were killed because of this and both variations cannot procreate with each other.
Films — Animated
Any of the many, many, derivatives of "The Little Mermaid" always use "The Little Mermaid Method" of course.
The most famous being Disney's, which features a time limitation (three days), a price (like always, her voice), and a clause which means Ariel turns into a polyp if she can't get Eric to kiss her in time. Note that this is all due to Ursula's plot; it's not a limitation of the spell (either that or Triton has more powerful magic).
There was also Ursula herself, who had an octopus bottom despite supposedly being related to Ariel and Triton.
Ursula being Ariel's aunt was left out of the final film and the sequel seems to follow this with her sister Morgana having the same tentacle bottom.
Interestingly, the mermaids in Disney's Peter Pan are of the "dangerously amoral" type, especially for its time. Their shell bras are much more "realistic" than Ariel's, which comes later; they're not held on by any straps, they're just sea-creatures attached to the mermaids' chests. One of them is quite obviously only covered by a flower lei, and another by her hair. When Wendy meets them, they attack her. When Peter calls them on this, one of them protests, "We were only trying to drown her!" By the time The Little Mermaid came around, Disney had toned down its mermaids quite a bit, and no one seems to remember the ones from Peter Pan.
Splash, obviously. Daryl Hannah plays the mermaid in question and much hijinx comes from her trying to pass herself as a human to her love interest (played by Tom Hanks). She transforms whenever her legs get wet.
The film version of Aquamarine follows "The Splash Method" - but see Literature below. She also has to turn back into a mermaid after sunset so she takes up residence in the town's water tower each night. She's also able to grant a wish to any human that helps her. As long as "it doesn't violate the laws of physics".
Made-for-TV MovieSabrina, Down Under has a merperson colony. It follows "The Little Mermaid Method"; no payment but there is a time limitation on Sabrina's spell which turns merman Barnaby human.
Dagon starts with the protagonist having a dream about a classic mermaid, only that it turns out she had sharp teeth. Later in the movie, he meets that mermaid... but she does not look like a typical mermaid or have the fangs she had in the dream. She looks like a human with gills along her ribs and a long, squid-like tentacle in place of each leg.
She was also his half-sister and wanted him to marry her. Yay Lovecraft.
The Made-for-PAX-TV Movie Mermaids (not to be confused with the 1980s Cher vehicle - which has nothing to do with mermaids) features the "Splash Method," with an added complication: the protagonists also turn into mermaids at low tide every night. They're also bound by some kind of natural law to grant wishes for humans. If a human says "I wish" within their presence, they're forced to grant it.
The 2003 film version of Peter Pan featured mermaids that look to simply be beautiful half-naked women with fish-tails; you don't notice the claws and the needle-sharp teeth at first.
The live-action film Hook, which has Peter Pan revisiting Neverland as an adult, has a brief scene with a trio of mermaids. They appear as half-beautiful woman, half-fish creatures with brightly-colored hair, each giving Peter a kiss to give him air before sending him up to the Lost Boys' camp in a giant oyster.
Ben Stiller appears as a merman in a TV commercial from Zoolander.
"She Creature" (aka "Mermaid Chronicles Part 1: She Creature") is an example of evil and ugly (sometimes) merfolk.
The Made-for-TV MovieThe Thirteenth Year has its mermaids and mermen appear as normal human children up until their 13th birthday, upon which they begin to go through a fishy puberty where they acquire a mishmash of marine animal abilities. In addition to becoming able to swim very well and hold his breath underwater for several minutes, the lead character starts to grow scales on his arms, discovers he can scale walls thanks to octopus powers, and discovers he can produce electricity thanks to electric eel heritage. And this is all before his legs become a fishy tail. That still doesn't explain why he swims like a dolphin rather than any normal stroke (he is on his high school swim team) before the transformation.
Pirates of the Caribbean features pretty naked women with fishtails...who also have fangs and will rape you (given that there's no men, it's apparently the only way they can supply their numbers) and eat you until there's nothing left. If you're lucky, you drown before the latter happens. But ignoring that, they can be very nice.
They are also strong enough that in large groups they can tear an entire wooden ship apart in moments.
Their fins can turn into legs if on land, but they don't do this very often, so they aren't very good at at it. Also, despite being aquatic creatures, they need air to breath. But will apparently dry out and die if left out of water for too long.
They can also shoot seaweed out of their hands like Spider-Man.
There's a running joke in The Cabin in the Woods about one character's desire to see a "merman." His partner replies, "Why? Those things are terrifying." He does eventually get to see one... shortly before it eats him, causing him to let out an exasperated "oh, come on!" as his Famous Last Words.
Hans Christian Andersen's "The Little Mermaid", of course. In the original tale, mermaids lived three hundred years before turning into sea foam; they did not have immortal souls and could not acquire one except through marriage to a human, which would give them the right to share in the human destiny. The Sea Witch required her most precious possession - her unusually fine voice - as payment for the transformation spell, since it required the Witch's own blood, and the voice was not refundable; the Sea Witch cut out her tongue. The transformation was very painful, as though she had been cut with a sword, and she was warned up front that forever afterward, every step she took would feel as though she were treading on knives. She was also warned that if the prince married someone else, her heart would break and she would turn to sea foam just as if she had lived out her full three hundred years.
At the VERY end, the mermaid who wanted a soul dies and becomes an air spirit who can earn a soul by 100 years of good deeds - with the Anvilicious remark that an air spirit's time may be reduced if she sees well-behaved children who make her smile, or increased if she sees wicked children who make her weep.
Arguably, the Disney version is the more sadistic and power-hungry version of the sea-witch, placing not only a time limit on the spell, but planning to ruin the mermaid's chance at happiness herself and steal the kingdom to boot. The original witch is neither good nor evil, and warns the mermaid of the consequences presented by the deal she wishes to make.
The witch provides the only way out of the spell to the mermaid's concerned sisters: a special knife for her to kill the prince in order to splash his heart's blood over her legs, reverting them to their fishtail form. The knife was much cheaper — all of her sisters' hair!
Halvgudene is a quite weird example. They have both women and men, that are all siblings and they vote forth their new leader which they call Mother or Father based on their gender. They are are a little more like Fish People due to having gills on their necks and have fish-like eyes.
Donna Jo Napoli's novel Sirena portrays mermaids in the traditional beautiful, sailor-seducing manner but adds a few details. Her mermaids (and their ocean-dwelling brothers mermen) are the product of the god Eros having sex with a tropical fish. They become immortal if they mate with humans.
In the Ingo Young Adult books by Helen Dunmore, the Mer (don't call them Mermaids, Mermen, or Merfolk) are described as half-human, half-seal. They can't become human but humans can become Mer apparently. They don't like humans very much, except for the Half Human Hybrids with whom they can communicate.
In Four Kids, Three Cats, Two Cows, One Witch (Maybe) Kevin tells Beverly a story about a family of merpeople where the father gets fed up of being neglected by his family and leaves to marry a human woman. He loses his tail because of this but is able to regain it at night time.
The original, printed page Aquamarine, by Alice Hoffman, centers around a Mermaid who is stranded in a swimming pool after a storm. She's a bit self-centered, but eventually realises she will need the help of the only humans who know about her if she is ever to return home and to her sisters. She cannot transform into a human at all. The rescue actually takes some time and effort — enough time for Aquamarine to develop a crush on a human man...
There's also Indigo, also by Helen Dunmore, which is about two Half-Human Hybrid boys, who have webbing between their fingers and toes, can hold their breath for a very long time, drink salt water, love eating fish, and constantly daydream about the ocean. Their mother was a traditional mermaid with a tail instead of legs, however.
It's also worth mentioning they have pockets in their tail.
Actually, only a "semi-mer", with one mermaid and one human parent, can become a mermaid when totally immersed in water, and will become human again when not, wet or dry; they can even take showers and baths, evidenced by the protagonist, who didn't learn she was a mermaid until taking a swimming class at the age of 13; full mermaids are permanently mermaids and cannot even become a human by use of magic, as King Neptune, who at one time loved a human woman, could not even transform her, though she desperately wished to go into the sea with him; this, actually, is her downfall, as she dives deep into the sea, believing Neptune's love for her, and her's for him, will allow her to live beneath the ocean, however, she drowns, which is one reason the Sea King gains a strong hatred of humans, banning mer-human relationships (which explains why Emily's father, a merman, is in prison in the first novel)
MaryJanice Davidsons' Fred the Mermaid erotica/paranormal romance has Fredericka Bimm as a lead character. She and the other mermaids in the series can simply become human at will. Even their human form is much faster and stronger than a normal human and they are all telepathic except for Frederika who is only teklepathic underwater.
In Harry Potter merpeople appear during the Second Task of the Triwizard Tournament, when Harry and the other Champions have to visit their village in the school's lake. The merpeople Harry meets are rather ugly by human standards, with gray skin, green hair and yellow teeth and eyes. The spin-off media book Fantastic Creatures And Where To Find Them mentions that there are at least three kind of mermaids—the Scottish selkies that Harry sees, the Irish merrow and the warm-water sirens)—and that the latter apparently fit the more traditional, "beautiful" model.
You don't get much more different than Feejee the Mermaid (heh) in Tales of MU. She can change at will between three forms: standard lady-half/fishie-half mermaid, an intermediate form with a distinctly humanoid lower half covered in scales, and fully human-like. She can also assume a sort of fighting form by growing scale-armor all over her body and claws on her hands. It's implied she can also change her face to a... less appealing form. Oh, and she eats people.
In fact, it's fairly strongly implied that merfolk in that universe basically are intelligent predatory fish with the magical ability to take on shapes more appealing to humanoids as a lure! They even have a myth about their goddess inspiring the land-dwellers to build ships for the express purpose of providing them with food...
Shapeshifting allows Feegee to overcome the Mermaid Problem with her boyfriend, though only for recreation. Mermaids actually reproduce in a more fishy way. They spawn.
H.P. Lovecraft's enigmatic Deep Ones are supposed to be the truth-behind-the-myth of mermaids. They're immortal, extremely unattractive... and they can mate with humans. The offspring are born effectively human, but undergo a slow metamorphosis.
Humans are willing to do this for huge amounts of gold, sharing of knowledge of Black Magic, and because the offspring never grow old (Carrying on my family by making immortal kids? I'll put a bag over her head!)
Worth noting as well that this is something of a bait and switch deal. The humans are given treasures and powers in exchange for minor deeds up until the Deep Ones are sure that their chosen human is completely dependent on them, then comes the proposition.
Deep One hybrids can retain their human appearance for decades before undergoing the transformation, long enough for themselves to interbreed with humans and pass along the Deep One gene.
Peter S. Beagle wrote a short story called Salt Wine where a merman rewards a sailor who rescued him by giving him the recipe for salt wine. It makes him rich, but then it turns out that a small number of those who drink it become transformed into mer-creatures themselves. Here, mermaids are portrayed as wild and inhuman, and they range from supernaturally hideous to supernaturally beautiful.
John Ringo's Emerald Sea features mermen and mermaids created by genetic engineering. They look like traditional merfolk but have a unique physiology combining fish and dolphin traits. They also find it hard to survive in the ocean after the loss of technology reduces them to a stone-age hunter-gatherer society.
Another very different—though friendlier—variation are Vonda McIntyre's "divers", who appear in several of her works. Imagine people with some sea-lion-like traitsengineered in, plus lungs modified for use in either air or water. That's the short description. (They're also usually described as attractive...and decidedly not subject to the Mermaid Problem.)
McIntyre's historical fantasy novel The Moon and the Sun has the sea people, who have hind limbs adapted for swimming instead of fish tails, as well as webbed fingers and claws. They also have an anatomical adaptation (just like that of the divers, only they come by it naturally rather than being genetically engineered) that allows them to breathe water as well as air. And their language consists of songs.
Piers Anthony's Xanth series does the Voluntary Shapeshifting route, while his standalone book Mercycle uses genetically modified descendants of normal humans, in whom the structure of human legs (complete with * ahem* equipment) is hidden within the tail.
Thea, in Mute, is a mutant with legs fused from the knee down and flipperlike feet. And yes, she averts the Mermaid Problem quite handily.
The post-apocalyptic mermaids in Alida Van Gores' Mermaid's Song come in two varieties: Mirra (traditional) and Mog (more fish-like). The Mirra have dolphin-like tails (and reproductive organs) and constitute an oppressed minority. Unusually, neither species can transform, since humans appear to be extinct and the surface world doesn't figure into the story much at all.
The Grindylow from the same novel are much less friendly mermen.
In Kai Meyer's first installment of the Dark Reflections Trilogy, The Water Mirror, the mermaids who inhabit Venice appear to be normal-looking women with fish tails- until you notice that their mouths are larger, longer, and filled with very sharp teeth. They are sentient but are considered bestial by the Venetians, who use them to pull their gondolas and ocassionally eat them. They cannot assume human form through magical means, but one mermaid lives as a human thanks to surgically-created legs and a mask covering the lower half of her face.
In Paul Jennings's short story Nails, merfolk can interbreed with humans, but the offspring look perfectly human up until their teens. Then their fingernails and toenails start to apparently multiply... these are actually developing scales, and when the process is complete, the hybrid has become a new merperson, legs fusing together into a tail which, like their arms, is covered in scales.
To be more exact, the mermen have legs but are covered in scales up to their necks while mermaids are just traditional merfolk, so only a female half-breed's legs would fuse.
K?b? Abe's short story Ningyoden (Mermaid Legend) is about a man falling in love with a flesh-eating mermaid. There's also some cloning involved. Since his work was influenced by Franz Kafka it's also a Mind Screw.
The Umiau from Chalker's Well World series are an alien species of aquatic mammals that just happen to look remarkably like traditional mermaids. (They're also Hermaphrodites, which sure solves the Mermaid Problem. Another aquatic race resembles sea lions.
Mermaids in the River of Dancing Gods series are half human and half dolphin but their social organization resembles a pack of hyenas crossed with the mafia. They make their living extorting protection money from fishermen.
L. Sprague de Camp features mermaids in several fantasy stories. In all of them (even ones in different continuities) the mermaids are part dolphin, rather than part fish. They are also streamlined for swimming, so the females breasts are generally smaller than those typically portrayed in mermaid art. The mermaids are fairly friendly: in one story ("Nothing in the Rules") one is even paid to enter a swim meet.
Merfolk appear briefly in The Chronicles of Narnia series. The ones on the coast of Narnia can leave the water for short periods and attend the coronation at the end of The Lion, the Witch, and the Wardrobe; later, out in the deep oceans in The Voyage of the Dawn Treader, Lucy encounters a wilder breed who (they deduce) stay submerged all the time. Notably in the latter case C. S. Lewis points out that from the point of view of underwater people, mountains and valleys' characteristics are reversed - mountains are the warm civilised areas near the surface, valleys are the mysterious dark areas populated by monsters. Another difference between the sea people that live near the coast of Narnia and the ones in the deep oceans is that coastal merfolk are portrayed as traditional mermaids and mermen with human heads, arms, and torsos, and long green fishtails beelow the waist, with the merfolk of the Eastern Sea are bipedal with regular human legs. (At least according to the illustrations.).
Alastair Reynolds wrote about "Denizens" in his Revelation Space series; the Denizens were created by genetic engineering and are thorough fusions of human and fish DNA, along with sequences to secrete antifreeze and let them breathe hydrogen sulfide instead of oxygen. They look thoroughly monstrous.
In The Merman's Children by Poul Anderson merfolk are humanoid, with bluegreen skin, webbed hands and feet, gills and attractive enough that one of them seduces a human woman and has children by her.
In the Doctor Who tie-in spin off Genius Loci one of the characters tells Bernice Summerfield a gruesome mermaid story in which a fisherman, with a fine sense of the pragmatic, chops a mermaid in two and takes the fish half home as his catch of the day. The bifurcated mermaid turns out to have been the daughter of the queen of the mermaids and hilarity ensues.
Goosebumps: Deep Trouble - The main character is rescued by a mermaid before it is captured and almost sold to a zoo by the mean humans.
In Andrei Belianin's Thief of Baghdad, the main character (Fish Out of Temporal Water with Laser-Guided Amnesia) and his friend Nasreddin encounter a mermaid, who will only help them if one of them satisfies her. The main character, recognizing the Mermaid Problem promptly passes the "honor" to Nasreddin. After some time, Nasreddin returns with a smile. When asked, he is surprised that his companion doesn't know that mermaids briefly turn into humans when they want to "get it on".
Kit Whitfield's Deepmen in In Great Waters are air breathing (although they can stay under for up to 30 minutes) mammals with tails, notably less intelligent than humans, although they have a language it's limited to purely practical matters, abstract concepts like religion being alien to them and can breed with humans. In fact all the royal houses of Europe (and possibly the world) have some Deepman blood.
"The Mermaid's Madness" by Jim C. Hines features a proud tribe of merfolk (they prefer the term "Undine") who appear to be of the standard human-on-top, fish-on-bottom variety. Members of the nobility of this tribe differ however, in that they have two tails (bypassing the Mermaid Problem quite nicely.)
In Robert E. Howard's Conan the Barbarian story "The Pool of the Black Ones", the arrival of a man on the deck of a ship at sea raises the question whether he's a merman. No, it's Conan. He's just been swimming for a while.
In a fantasy book called Elsewhere, mermaids apparently swim in the ocean that's in between the world of the living and the dead (It Makes Sense in Context). They briefly appear when the main character is trapped under the water, unable to move or breathe. They are beautiful, but vain and meanspirited, making fun of how ugly she is before leaving her to drown. They seem more like the Fair Folk than evil though.
Sarah Porter's Lost Voices Trilogy has a rather dark and unique take on mermaids. Certain young human girls have the potential to come back as mermaids, if they suffered severe abuse or neglect in their human lives. They are reborn as beautiful mermaids with fish tails and enchanting voices that can entrance or madden humans - though the talent is actually pretty rare considering. These mermaids are ageless, though not completely immortal, and all still traumatized from what they endured in life. They also act a bit like water mammals - they do breath air, but they can hold it for a very long time underwater. They sink ships and drown people as revenge against all of humanity for what was done to them by parents/caretakers and other people in their previous lives - this is also implied to be something like an instinct. They form tribes in the oceans (this is for survival, as they can still be killed by humans, sharks, etc. and still need to eat, though they can survive on raw shellfish) and have strict laws against having any contact with humans other than singing them to their doom. Once transformed, they can't survive out of the water for long, and if trapped on dry land, they will revert to human form and then die(a very painful process). It is implied that certain young men and boys have the same kind of potential if badly treated in life, but attempts to transform them are usually unsuccessful.
In L. Jagi Lamplighter's Prospero Regained, several mermaids play minor roles. We are told that one recovered Hector of Troy's sword, Duranadel, to be given to Roland.
The Ustredi in The Chronicles of Magravandias. They are not dissimilar to The Fair Folk. Some are just straight-up Fish People and some are so beautiful it hurts to look at them. The beautiful ones are the most dangerous. The Palindrake family has the ability to command the Ustredi through means of an ancient contract.
Mermaids appear in the third book in Michael Scott's The Secrets of the Immortal Nicholas Flamel "The Sorceress". They are the daughters of Nereus, the Old Man of the Sea who has the octopus bottom but they all have fish tails. Josh notes that some are beautiful women while others resemble fish and crabs. They all have green skin and sharp teeth and claws (hint: they're not on the side of good) as well as having apparent powers through singing. Virginia Dare is attacked by one and claims to have stolen her voice, leaving her mute and then implies Nereus will "dispose of her" since she is useless to him now.
In the Kathi Appelt novel Keeper, a young boy named Jacques is a merman; he changes to his form via the "Splash" method.
Anne Greenwood Brown's young adult novel Lies Beneath and its sequel Deep Betrayal feature mermaids who can change into human form at will (though depending on how long they've been out of water, immersion can trigger an immediate, instinctive change, and it is sometimes painful). In mermaid form, a silver ring is visible around their neck. Other abilities include generating electricity (which can be governed by emotions), telepathy with fish, and a telepathic bond with others in their family while in mermaid form, which can only be severed by the head of their family. Mermaids are instinctively drawn to others in the link, and it makes it very hard for a mermaid (or merman) to strike out on their own, though some with stronger wills are capable of doing so. They age at a rate of one year for every three that go by. Mermaids are unable to break a promise once made, and react VERY badly to humans who do so.
Feeding-wise, Lies Beneath mermaids can eat normal food, but are drawn to humans whose auras shine with positive emotions, absorbing them directly from the human, leaving them as dried-out husks and providing the mermaid with a brief reprieve from their natural bleakness. They are predators, and act like them.
Reproduction is done in two ways: first, directly with humans while in human form. After the birth, the infant is left with the father until they learn to walk, at which point they are returned to the mother, and live as a mermaid full-time. The other, less common way is known as "reinvigoration", in which a human drowns and has their heart restarted via the mermaid's electric shock. This method fully converts the human into a mermaid, and bonds them to their new family. It is a very rare event, but can be done, usually by a mermaid who had drowned a victim and had second thoughts.
In the short story National Geographic on Assignment: Mermaids of the Old West (included in the anthology Somewhere Beneath Those Waves, different species of mermaids correspond to different species of fish, including shark and catfish.
Dream Girl by J. Conway Jameson features a lesbian mermaid named Áine, who seduces and kills unconvicted rapists, murderers, and abusers of women. She uses "The Splash Method", but taking after the Nixie legend, her human form has a perpetually wet skirt or pant hem.
The Australian series H2O: Just Add Water is about three girls who, after finding their way to a mysterious island, turn into mermaids following "The Splash Method" after swimming in a Magical Pool during a full moon. They also have powers over water: one can freeze it at will, one a can boil it at will, and the other essentially is a water bender (also she can multiply it from a drop to a bucket).
The Man From Atlantis looks like a human but has webbed digits, can breathe underwater, and can withstand extreme depths and pressures.
In one episode of Red Dwarf when the characters are in a Virtual Reality Program Cat creates himself a Mermaid girlfriend. She's a reverse mermaid with the fish part on top.
Holly: Somehow, I'd imagined she'd be a woman on the top and a fish on the bottom. Cat: No, that's the stupid way around!
Night Gallery featured the episode "Lindemann's Catch", which featured a fishing captain growing captivated by the mermaid he caught. Desperate, he finds a way to give her legs, so she can live on land; the only problem is that she also gets a fish's head and gills as well.
Way before either of those, Dengeki Sentai Changeman had a mermaid-themed hero, though unlike Madison she didn't have a mermaid mecha (she had to share a mech with Change Griffin).
In Round the Twist, "Nails" Linda falls in love with a boy who seemingly has a strange disease which causes him to grow extra fingernails and lose the use of his legs. It turns out he is slowly (and irreversibly) turning into a merman as his mother is a mermaid. This was, like all of the episodes in the first two seasons, based on a Paul Jennings short story- in this case, Nails.
One episode of So Weird features a merman who turns human at will and can remain human even in water, though he's got webbed fingers. There's some sense of urgency to his plan to seduce Molly, which may or may not point to a time limit. He also appears to have the power to turn humans into merfolk.
A weirder, later episode featured a man who is implied to have turned into a merman by force of will - the valley he lived in was flooded and he refused to leave. By sheer determination, he gained gills and webbed hands so he could remain in his house.
A "mermaid" (technically, one of a group of people genetically-modified to survive deep underwater) appeared in an episode of seaQuest DSV. She was of the bordeline-Cute Monster Girl variety; she didn't have any blatant sea-creature features but sported a set of gills along her rib-cage and webbed digits. She also felt very uncomfortable outside the water. And, out of all the people onboard Sea-Quest, she got along the very best with Darwin.
Charmed mermaids have "hearts as cold as the ocean". Phoebe was turned into one. They are also immortal. They can live for hundreds of years but they really don't care since they have cold hearts. They also can't turn into humans willingly. One makes a deal with a sea witch to get legs for 30 days to find love. While this is happening she still turns back into her normal form whenever her legs get wet. A human man professing his love for a mermaid turns her human permanently.
One of the (supposedly) main characters of Sanctuary is a mermaid of the human head/fish tail type. We don't know if she is able to transform, but not likely considering her habitat is water as is that of her race. Also have a type of telepathy to communicate with the members
Dark Angel': "Gill Girl" featured genetically-engineered mermaid and merman equivalents. They looked mostly human, but had the gills of fish and the echolocation of dolphins.
The Torchwood episode "From out of the Rain" featured Pearl, said by the Ghostmaker to be "the closest thing you'll ever see to a living mermaid". She didn't have a tail or gills, but could live in water, glistened like she was wet all the time, and liked to drink tears.
Kamen Rider Kiva features Basshaa, the Last of His Kind of a race of Gill Man-like creatures, who usually assumes the form of a young boy named Ramon. Thanks to a pact with the title character's father, he can loan his power to Kiva, turning the Rider into a Glass Cannon armed with a gun that fires high-power water bullets (which also causes Kiva to act somewhat childish, like Basshaa/Ramon). The Mermen are also a One-Gender Race, with the backstory mentioning a Mermaid race that resembles the traditional mermaid, and that the two races have to mate with one another.
In the second series of Primeval, the team encounter a race of aggressive futuristic sea creatures bearing resemblance to a cross between baboons and elephant seals. Word of God reveals the creatures are called "Mer" and they're descended from primates. To support this theory, they have well developed vocal chords and arms rather than flippers. They also sing, although to Connor it sounds more like something he once heard in Glastonbury in a "chill-out" tent.
A new MockumentaryMermaids The Body Found is about merfolk that are the fourth or fifth (depending on whether or not the "Killer Chimpanzee" is real and an actual seperate species) of the descendants of the common chimpanzee/bonobo/ancestor.
Mermaids appear in two separate episodes of Sabrina the Teenage Witch as minor characters. In one episode Sabrina has to take over Mercury's deliveries and delivers the wrong package to a mermaid - a pair of high heels so presumably she can't turn into a human. In a seventh season episode Sabrina finds a man who has a mermaid trapped in a cage in his basement as part of his collection. He allows her a small bathtub.
In one of the novelizations "Age of Aquariums", Sabrina finds a race of merpeople inside the school's new aquarium called the Keftiu. It turns out that it's actually the shrunken lost city of Atlantis. At first when Sabrina visits them, she simply stays inside a large bubble of air. The second time, she turns herself into a mermaid so they'll trust her and pretends to be their mythological "Conch Queen of Doom".
Safe Havens has Remora, who has taught us a lot of unexpected things about merfolk over the years. For example, they use bubbles for currency. Not sure what they use for a wallet.
They also have the option of either being a merperson 24 hours a day or splitting their time between being a human for half the day and a fish the other half.
In some variations of Nautical Folklore, it is possible to take a mermaid captive by stealing an item of magic thus keeping her captive. Because Abduction Is Love she will often marry the man who did this and raise a loving family. But, she will spend all her spare time trying to find it, and once found she will swim away.
Those who are nice to stranded mermaids on the other hand receive kindness from them. One Scottish boatbuilder rescued a stranded mermaid and wished that no boat he built would ever sink. His family's boats are, according to one version, famous through large parts of Scotland.
Rifts manages to pull off just about all Mer-types. In addition to the traditional mermaid/man, you also have random aquatic species, and the Amphibs, human mutants who range in appearance from Black Lagoon-like to Fish-headed.
Changeling The Dreaming has the Seelie Merfolk and the Unseelie Murdhuacha (pronounced mer-RU-ka). The Merfolk are what one generally thinks about when one thinks mermaid: their lower halfs are of various bony fish (except for House Melsinee, who instead take the form of air-breathing marine mammals and reptiles). The Murdhuacha are merged with crustaceans, mollusks, and other seagoing invertebrates. When either Kith takes to land, their lower halves automatically turn into legs and they resemble Sidhe with their otherworldly beauty.
There's also a bit of horror, as the Merfolk and Murdhuacha are trying to fight off the game's constantly-oppressive force of Banality. Any changeling who succumbs to Banality loses all access to their fae abilities and forgets all about their second life. Imagine being one of them, and coming to in the middle of the Atlantic Ocean... about five hundred feet down.
The New World of Darkness variant, Changeling: The Lost, has the Swimmer Skin type of Changeling. They can breathe underwater with a point of Glamour (their "mana", powered by dreams), but cannot breathe air unless they spend another point of Glamour or let the first power expire. There's also an Elemental version, but those aren't Merfolk.
There are merfolk in Dungeons & Dragons, but there's nothing that really distinguishes them from your standard merfolk model aside from a slight tendency to advance in the bard class.
Mystara, like most D&D settings, has merfolk, called "merrow". Its are unusual in that they can breed with humans (solving the Mermaid Problem via shapechanging by one or the other). The Queen of Aquas, half-sister of the heir to the Empire of Alphatia, is half-merrow on her mother's side.
Dungeons & Dragons also has sea-elves, sea-ogres (merrow), sea-trolls (scrags), sea-ghouls (lacedons), tritons (who are distinct from merfolk) and so on and so forth. Also the sahuagin (shark-men), locathath (generically fishy humanoids), kuo-toa (Lovecraftian Deep Ones), and about a dozen more. Essentially, all the above variants and then some.
A better rule is "If there is a race that walks on land, there's an aquatic variant. If there isn't, here's the template to make 'em."
Cthulhutech has the same Deep Ones as anything else Lovecraft-related, but this time they brainwash small settlements to produce offspring, rather than their standard dependence method. And now they're out to find Cthulhu.
Dolls in the "Merwees" collection (somewhat similar to Polly Pocket, except the dolls were about 3x the size) would change from "human" to "mermaid" when in warm water. They could be changed back (to resemble a human woman wearing leggings) by running them under cold water, or simply leaving them out.
There's no limitations on Donald's easily reversible spell that allows Team Keyblade to go to Atlantica in the Kingdom Hearts games - but it does turn Sora, Donald and Goofy into different merfolk than Ariel and Triton. They become human/dolphin, duck/octopus, and whatever the hell Goofy is/turtle.
The Legend of Zelda has the Zoras. They have legs with flippers, sea resistant skin and can breathe underwater. They have the tail of the aquatic animal they're based on instead of hair. They are able to live on land, but can dehydrate quickly.
It's heavily implied that one of the Rito (evolved Zoras) in The Legend of Zelda: The Wind Waker had a human ancestor. There must have been a marriage somewhere in the line, though it's unclear whether it happened before the Zora became Rito or afterwards.
Less pleasant are the "River Zoras" (as they are called in Oracle of Ages), also known as "Zolas" due to a difference in translation, which are green, fish-headed and pop out of the water to shoot fireballs at you. Technically the River Zoras predate the friendlier Sea Zoras, but the latter became more popular.
The Mermen race in Darkstalkers are an odd mix of the traditional beautiful merpeople, the Creature from the Black Lagoon, and Lovecraftian Deep Ones. They have disturbingly beautiful faces, webbed claws for feet and hands, come in a multitude of rainbow colors, and can transform their body parts to mimic the appearance and abilities of any other sea creature - although usually on a bigger scale.
In Mega Man 9, Splash Woman's look is based on the regular mermaid—female human on top/fish tail on the bottom. She sings to call in a few waves of fish robots and uses a laser trident.
In E.V.O.: Search for Eden, you are able to become a mermaid, albeit only temporarily, in the last age by stepping into the ocean off the southmost point of South America. While you're first given a workable amphibian version of whatever you've evolved into, you now have the option to "evolve hands and feet"; doing so will turn you into a seal, then a dolphin, then... something the Let's Play titled "Abomination" for a good reason◊, and then blonde mermaid. It's not clear which gender you are, but you apparently attack with devastating effect by kissing your foes. Also, since you cannot evolve further, you cannot replenish HP by minor evolutions, making the boss fight a good deal more difficult. Of course, since you're an evolutionary dead end, once you've defeated the local boss you're returned to the shore as the creature you entered with.
In Harvest Moon DS, one of the special girlfriends is a mermaid. Or at least she would be special if they didn't explain how to get her in the manual. She can't transform and if you want to marry her (yes, you can marry her) you have to build a pond on your farm first.
Rune Factory 3 of the Rune Factory spinoff series features Persia, who transforms by the splash method.
In Chrono Cross, mermaids (but no known mermen) are considered ordinary, albeit aquatic, demi-humans. They do not transform on land but can still somehow transport themselves as though walking (necessary for the playable character Irenes), and they can have children with humans (in fact, two of the other playable characters are Irenes's nephew and niece.) The children appear perfectly human (although the son is mentioned to be inhumanly beautiful). No explanation for any of this is even attempted.
Vaporeon is a Pokémon based on a mythical creature in Singapore folklore called a "Merlion" (Think upper body of a lion and lower body of a fish). It's also a walking Visual Pun in the sense that it's a feline-like creature with fish-like traits. In other words, either a "Catfish" or a "Sea Lion".
Warcraft has the Naga, the cursed elves so transformed at the Sundering. They don't have fish tails per se, but instead have serpentine lower bodies and can breathe underwater The multi-armed females are relatively cute as per normal, but males are far more draconian in apperance.
Kingdom of Loathing has the Mer-kin, a race of nasty, xenophobic merfolk who worship Lovecraftian horrors.
It should be noted that the word "merkin" has a real-world meaning. It's a pubic wig. No, really.
Quest for Glory V has the Merfolk, but they are just the standard fishy bottom, human top variety.
The King's Quest series has two mermaids: one in King's Quest II and the other in King's Quest V The first's colouring bears an uncanny resemblance to Disney's Ariel, whilst the second is blonde and friends with a beach-hermit. Both are voiceless and have minor roles in the game.
In Overlord II Mermaids appear as enemies. They're all rather fat compared to other examples.
Tales of Monkey Island has the Vaycalians, a race of mer-people that live in Spinner's cay and left some ancient artifacts across some islands. They are said to have stopped their evolution years ago, and used their technology to give themselves human feet to live on land, but eventually reversed it and returned to the sea. The Vaycalians also have no visible secondary nor Tertiary Sexual Characteristics, making it impossible to tell if they're male or female, which leads to some awkward moments between the flirty Vaycalians and the happily married Guybrush.
Not to mention like Real-life Carribean-located water-dwellers, they are very colorful.
SaGa Frontier has Mesarathim, a grey-skinned mermaid who, like Irenes in Chrono Cross, spends more time out of water than in, although she'd like to change this.
There are merpersons in Dwarf Fortress. Their bones used to be worth as much as dragon bone, though now mermaids (as sentients) are no longer butcherable and don't have valuable bones.
For a while, players were bypassing the "no butchering sentients" rule to get valuable mer-bone anyway, by building "mermaid farm" machines that would trap merfolk until they bred, then beach the offspring so they died and their bones rotted clean.Even Toady thought this was too much, and tooled down the value of mer-bone.
Final Fantasy uses typical girls-with-fishy-bottoms mermaids as NPCs in the first game, but somewhere along the way someone decided that they may have been a bit too much of a cliche fantasy element and they haven't appeared in a game since (unless you count the totally-not-Gungans Hypello in Final Fantasy X.)
Mermen in Battle for Wesnoth, even if they're doomed to the supplementary in most campaigns, still are a major playable race. A good spectrum of units (3 branched advancement trees covering all main roles) fast and powerful in their own environment, but weakened on the ground.
The mermaid we meet in the Telltale Sam & Max games is completely human in appearance (save for oddly-coloured hair), and (by Word of God) basically immortal, except for accidents. She's also psychotically evil, but that's probably not a mermaid thing.
She's not really a mermaid either. Her outfit has a distinct mermaid theme to it, but that's neither here nor there. She's actualy an unholy golem created from an infernal recipe called "The Cake Of The Damned", and why the ambulatory form of The Cake Of The Damned should be a psychotically evil, attractive young lady in a mermaid-themed outfit is for the writers to know and us to never find out.
At least, that appears to be the case until episode 305 apparently, and somewhat confusingly, Retcons the above, making her actually a mermaid.
Ōkamiden allows the player to have a mermaid girl as their partner. She looks like a traditional mermaid, but she has the Dragonians' fin-ears.
The Undines from Monster Rancher 2 have a transparent, Jell-like appearance and use both water and ice based attacks.
Etrian Odyssey: The Drowned City has mermaids appear in the form of the Deep Ones. The first of these you meet is their childlike Prince/Princess who can freely spawn them from drops of hir blood. Sorta creepy...
Nami the Tidecaller from League of Legends is a member of a mermaid race called the Marai. As well as being rather more fish-like than the usual mermaid (they have pale-skinned humanoid faces and arms, but at least their females have Absolute Cleavage coats of scales covering their Non-Mammal Mammaries) they also live so deep in the ocean that the light of the sun and moon cannot reach them and they know nothing of the surface world. Nami, at least, can cross land by floating on a small personal vortex of water.
Touhou has Wakasagihime, the fresh water mermaid of Misty Lake. She's normally a gentle Youkai who spends her time singing songs and picking up rocks, but something caused her to become violent in Double Dealing Character, where she serves as the first boss.
In Shantae: Risky's Revenge, mermaids appear as enemies in the aptly named Mermaid Cliffs. They look pretty standard save for the green skin, and they can shoot bubbles with their tridents and dive into puddles to avoid attacks. However, when they die, they shrivel up and turn into a tiny fish before disappearing. The title character eventually gains the ability to turn into a mermaid, and can shoot bubbles underwater.
SuikodenIV packs a mermaid that deviates from the standard fish tail mermaid. While on an island that you shipwreck on, you can meet a mermaid that has legs instead of a tail, but still is called a mermaid in game (by both game text and another character in your party). She can give items if you play nice with her, and also warns the party that the big boss of the island has attacked one of their own.
The 'Freemium' game Dawn Of The Dragons recently added Merfolk (both male and female). One Merfolk Unique Character has flavor text in her entry about her being captured by fishermen, shifting her tail to legs at will to escape the net, then being told to put her tail back as she "is more valuable with the tail whole".
Mermaids in Genjuu Ryodan can use music to put units into sleep and have the same movement range as foot units without needing to transform their tails into legs at all.
Mermaids replace the Dolphins in Something Else, but they only show up in the secret exit path for Mysterious Maze.
A common type of citizen in Wadanohara. Since they can't go to the surface due to not having legs, a few of them often ask Wadanohara for favors, like collecting apples.
Murms, which are joke creatures that are brightly-colored, generic mermaids that do nothing but flop about and look silly, yet are beautiful and harmless. They hide the monstrosities that are...
Menaces, which are monsters related to sirens that lure victims◊ into the water with their friendly words, where they tear them apart and feed on their hearts. They are known as "the true mermaids", and are extremely rare to spot.
In the original vesion of Elf Life, extremely talented mermaid queen Leukothea gets banished to the surface after fairy queen Glynthial ruins her ability to breathe water. When Airek, a young half-ogre accompanying them, addresses the Mermaid Problem, Glynthial points out that mermaids do have functional legs inside their tails which will come out now that she's on land... but as Thea's lived in the ocean her whole life, they'll be hideously stunted and misshapen. Turns out they're quite attractive, actually.
When the protagonists of Accidental Centaurs have to cross an ocean at one point, their helpful genie friend turns them into merfolk instead. (Humans cannot exist in otherspace, but mythological parallels are acceptable.) Alex is at first enthused at the opportunity for hot mermaid sex, until his girlfriend cruelly points out that fish use a different method for reproduction. (Also, the mermaid tails appear to have knees. WTF?)
Many artists commonly draw merfolk with tails that have knee and ankle joints instead of a spinal column. They'd probably swim much like a human using a monofin, a single large flipper to use both legs at once. If you watch swimming competitions you do notice that when they are underwater (after kicking off the end) they do keep their legs together and use an undulating motion. A horizontal fluke just extends this motion.
Pearls of Mer a webcomic about a standard, Splash style mermaid.
Rebecca Burg has released concept art which reveals that in Loonatic-land, merfolk are a type of aquatic vampire. When they're luring in prey, they're fairly cute, if slightly overfinned. When they're about to feed, their eyes take on a crazed look, and their mouth splits into a grin which reveals that all their teeth are razor-sharp. And unlike some vampires in this setting, where being turned isn't even a guarantee, when a mermaid feeds on you, you become one.
Homestuck's Troll race is divided between a land-dwelling caste and a sea-dwelling caste, the latter of which have vastly different internal anatomy from the others as well as a pair of fins sticking out from their necks. They are the royalty amongst the Trolls, and the only examples we see of them are the heir apparent and a genocidal noble with a crush on her (as well as any paradox-created ancestor/descendants of said two).
At the end of the first volume of Tales Of Gnosis College, Li Anwei reveals that she is in the process of a slow transformation into something like a mermaid by showing expanded webbing growing between her fingers. She still looks pretty human when she takes to the sea, though.
Deviant Art: http://spyra.deviantart.com/art/Symbiosis-30554745 Mermaids are the result of forlorn women jumping into the ocean to reunite with loved ones lost at sea. Either out of revenge or sympathy, the ocean created a new breed of fish that symbiotically joined bodies with the woman so they may survive underwater.
In The Dragon Wars Saga, the merfolk can shift their tails into legs — albeit still covered in scales — for use on land. Many of them live in freshwater and it's been suggested they use this form for migrations if their home dries up. However they become ill if out of the water too long.
Mermaid's Tail Galleries has galleries of mermaids based on specific fish (mostly sharks, rays, and freshwater fish) in addition to the usual generic mermaids.
In Aelan mythology from Ustal Naror islands, mermaids will be beings of eeriness that will drink blood
In American Dragon Jake Long mermaids can't switch but they can live on land for long periods. Long enough for one to go undercover at Jake's school for several months as his principal, using a wheelchair. As an additional feature, they seem to have a natural talent for maths. It also features a mermaid who is afraid of water.
The short lived series The Prince Of Atlantis feature a legged human-dolphinish-hybrid and a half-human-half-ray creature.
In an episode of Family Guy Lois is rescued by a reverse merman (human legs, fish head and torso).
Notably, the actual gag was a discussion of the Mermaid Problem. He argues that if he were a normal merman he wouldn't have the appropriate reproductive anatomy, and she argues that while they could technically have sex, it would just be weird.
Xiaolin Showdown has Dyris, who initially appears as an innocent variety of mermaid with the appearance of a beautiful maiden and a fish-like lower half. She later turns out to be an evil seductress who uses her feminine wiles to trick the Xiaolin monks into helping her plan to submerge the entire world. Also, when she steps on dry land, she spontaneously shapeshifts into a giant, amphibious monster, and returns to her original form when wet.
Milo Manara's City Hunters had Odysseus and a friend catching mermaids. It's seen that Odysseus seduces a traditional mermaid, and his friend "jumps on a grenade" with a reverse mermaid.
An episode of The Transformers took place on a planet of Human Aliens who could become mermaids by swimming in a magic pool. It later turns out that the pool allowed the aliens to actually take on any form they wished. This allowed Seaspray to become a merman (although with Transformer feet) and one of the mermaids to become a Transformer who took the form of a gondola. And then Rumble turned into a tree (Let us not forget that the old-school Transformers could get downright strange sometimes).
In an episode of The Adventures of Super Mario Bros. 3, Mario met a reverse mermaid named Holly Mackerel who was both unattractive (hence that episode's title, "The Ugly Mermaid") and not very bright (she thought Mario, wearing a Frog Suit, was an actual frog).
From The Super 6 we have Super Scuba's Girl Friday Bubbles, a mermaid who tags along with him on his underwater cases.
In the first season of Metalocalypse, Dethklok writes the song Murmaider. The video features traditional mermaids at war with considerably uglier, crustacean-themed mermaids.
Nathan: "This one's called Murmaider!"
Murderface: "It's about mermaid murder."
In one episode of ReBoot, Dot and Bob turn into shark-people to participate in a game. Unusually, their fin stroke horizontally, like true fish. Their bodies are also slender, and very long.
Bubble Guppies flips the mermaid concept on its head. The merpeople on the show are tiny, about the size of guppies. They can't change shape, but apparently they've developed the ability to swim through air.
Marina from the French cartoon Zig And Sharko is mostly a typical mermaid, except she can survive on land and walk on her tail fins. Also, her parents are a mermaid and a human man, somehow.
Applied to Iguana Seals in All There in the Manual in Avatar: The Last Airbender. Similar to the manatee-mermaid misidentification, the hybrid animals were often mistaken as mermaids by sailors due to their melodious sounds. Like the manatee, upon realizing they were nothing but seal-finned aquatic reptiles, they were subsequently killed for food.
Let's not forget Marina, Handy Smurf's Smurf-sized love interest.
Gravity Falls features Mermando, a merman who served as Mabel's Guy of the Week. Mermen seem to go through puberty early—at twelve he already has bits of stubble and a deep voice which he says changed when he was three—and have seventeen hearts. ("Horrifying but true!")
Sabrina: The Animated Series - Sabrina, Chloe and Salem cast a spell to spend one hour at the beach and end up getting transformed into mermaids - Salem becomes half cat-half fish. They're unable to stay out of the water for longer than a few minutes without dehydrating and the spell reverses itself within an hour. Provided the three of them are at the island where they arrived that is. Oh and Sabrina's fins are apparently powerful enough to outswim a torpedo.
P.T. Barnum's museum of curiosities featured a twisted, mummified specimen called a "Fiji Mermaid". It was, of course, a fake, created by sewing the front half of a monkey to the back half of a fish. It was also every bit as f'ugly as that combination sounds.
Robin Jarvis evidently took inspiration from it for the mummified mer-thing in The Whitby Child.
Manatees, Dugongs and Sea Cows were often mistaken for mermaids, perhaps because the females have breasts and float upright in the water to nurse their young. Taxonomists recognized this by assigning these species the order name Sirenia
There's a wonderful bit in one of Christopher Columbus's logs about seeing "mermaids" in the Caribbean - he says they're less beautiful than they are painted, because their face resembles that of a man. A seriously ugly man, one assumes.
There's the "other" reason they're compared to mermaids that's a prettier lie than the ugly truth as according to Manswers, their docile behavior and anatomy's similar to a human for both partners to "enjoy."
In West African mythology, mermaids (aka Maame Water) are beautiful but evil demonic spirits tasked by Satan to steal people's souls in exchange for riches. However foreign depictions of mermaids such as in Disney's The Little Mermaid are still popular because, after all, Their Mermaids Are Different.
An extremely rare human birth defect, in which both legs are fused together from crotch to feet, is known as sirenomelia or "mermaid syndrome". Sadly, people with this condition usually die within a day of birth, due to associated abnormalities of kidneys and bladder; to date, only a handful of sirenomelia patients have survived for long, and only with drastic surgical help.
The Annals of the Four Masters (a collection of medieval Irish histories compiled in the early 17th century) casually mention mermaids being caught several times. Most impressive is a mermaid listed as having washed ashore in the year 887:
One hundred and ninety five feet was her length, eighteen feet was the length of her hair, seven feet was the length of the fingers of her hand, seven feet also was the length of her nose; she was whiter than the swan all over.