Merpeople

Merpeople are a legendary species that are half human and half fish. Females are referred to as mermaids, while males are referred to as mermen.

They are featured most heavily in The Little Mermaid, and other related media. Other than The Little Mermaid franchise, they have made appearances in other films and shows.

Background
The earliest record of merpeople were the "fish people" created by the goddess Tiamat in the Babylonian Creation Epic the Enuma Elish, written on clay tablets several thousand years ago. They appear in the folklore of many cultures worldwide, sometimes associated with perilous events such as floods, storms, shipwrecks, and drownings. In other folk traditions, they can be benevolent or beneficent, bestowing boons or falling in love with humans. Some attributes of mermaids borrow influence from other mythological creatures, including the Sirens and Naiads of Greek mythology. In many stories, mermaids and mermen lure and attract humans with their enchantingly beautiful and melodic voices to send the humans to their demise or simply entertain them.

Physical appearance
Merpeople possess the traits of both humans and fish: their upper body is generally depicted as human or humanoid, while a fish tail replaces the lower body. In order to live underwater, merpeople have the ability to metabolize oxygen from the water, which negates the need to surface to breathe air. However, they are more than capable of breathing air outside of the water, as evidenced by certain events where they are on land for prolonged periods of time. Also, their tails move in a vertical fashion similar to dolphins instead of fish, allowing them to jump out of water and swim at such a speed as to rival most vessels during that time. Merpeople are often depicted with a degree of superhuman strength and abilities, which is evidenced by Ariel moving an underwater boulder from the grotto as well as knocking down a door with relative ease. They have a connection to their aquatic home, as they can communicate with aquatic and marine life.

The Little Mermaid franchise
In The Little Mermaid and related media, merpeople are shown to live in the sea within various underwater kingdoms. Atlantica is the most prominent, though others are implied to exist in the TV series. They are shown to coexist with sea life, with whom they are able to communicate. They generally do not interact with humans, due to the danger, though this changes after the events of the first film. It also shows that many unnamed ones are held as prisoners of Ursula. They presumably made bargains for her aid, and were unable to uphold their part of it. As a result, they are withered down to polyps. Ursula's death causes them to be restored to their natural forms.

Interestingly, dialogue from one fish character in the TV series implies that most fish either don't realize or don't know that merpeople have parts of their biology that resemble that of a human, as it refers to them as "half-fish, half-something else". Mermen generally appear bare-chested, but females wear a bikini top of some fashion. It is also possible that they are herbivores, or at the very least vegetarians, as they aren't seen eating fish, and apparently they don't seem to view doing so as being ethical.

Peter Pan
In Peter Pan, which was made before The Little Mermaid, the mermaids are similar in appearance to their later counterparts. The mermaids as a whole appear fun-loving, and do not seem to have any real cares or troubles besides Captain Hook. They live in Mermaid Lagoon in Neverland, and are shown to be very admiring of Peter Pan. As a result, they become jealous of Wendy, teasing her and even making an attempt to drown her. However, in the spin-off series Jake and the Never Land Pirates, a kind mermaid named Marina helps Jake, Cubby, Izzy, and Skully in some episodes. Also, Marina's younger sister Stormy is seen in some of the episodes Marina is in. Stormy has more of a personality of the 1953 mermaids. Also, in the episode "Treasure of the Tides", Izzy becomes one for a day, with some leaves.

Gravity Falls
Merpeople were briefly mentioned in "Dipper vs. Manliness" by the Manotaurs, and a merman named "Mermando" appears in "The Deep End". They are stated to live in the water. What may be a skeleton of one can be seen in the Mystery Shack gift shop, although it resembles the famous hoax Fiji Mermaid. In the opening credits, a picture of one like the one in the Mystery Shack can be seen at the end of the credits. They also appear in the episode "The Deep End" when Mabel Pines befriends a beautiful merman named Mermando. He had been swimming in the Gulf of Mexico, but was caught by a net from a fisherman. He later is seen at the Gravity Falls Pool. When she first saw him, she went all silly, wanting him. She soon realizes that he is a merman, and thus tries to help him, of which she succeeds. He is last seen swimming back to his home. Merpeople have about 17 hearts.

American Dragon: Jake Long
Merpeople are among the magical creatures featured in American Dragon: Jake Long. Two notable merpeople are Silver and Dolores Derceto.

Sofia the First
In the special Sofia the First: The Floating Palace, Sofia and her family visit Merroway Cove for a vacation and discovers an undersea kingdom homed to merpeople and makes friends with a mermaid named Oona. Ariel makes a special guest appearance. Thanks to Oona's help, Sofia can now turn into a mermaid using the Amulet of Avalor. Sofia turns into a mermaid again in Cool Hand Fluke and her mermaid friends also appear as well.

Star vs. the Forces of Evil
According to Mewni royalty law, a Princess cannot become queen if she cuts off a Mermaid's tail. Star was going to cut off a Mermaid tail when Marco stopped her from doing it. But instead Star throws the axes into the Mermaid's tank, broke the glass, and it ran out with the water.

Splash
In 1984, Disney released a live-action romantic comedy under the Touchstone label which was centered around a mermaid, played by Daryl Hannah. Taking the name Madison (and thus inspiring a great many real-world girls to do the same), she leaves the sea to seek out her beloved (played by Tom Hanks), whom she rescued from drowning. However, marine biologist Dr. Walter Kornbluth is convinced that mermaids are real, and will stop at nothing to protect his credibility by exposing her to the world.

The Thirteenth Year
In The Thirteenth Year, two merpeople are shown: the protagonist, Cody, and his biological mother. The ones in this film are somewhat similar in appearance to one from other media. However, there are several distinctions. They are able to pass for human until they turn 13, when the fish physiology surfaces. The transformation is shown to be triggered by exposure to water. In addition to the fish-like tail, they also have scales on their palms, and small fins that run along their forearms. They also seem to possess other abilities, such as the one to generate electricity, to the point that they can fire it similarly to a defibrillator, and the one to stick to surfaces.

The Chronicles of Narnia: The Lion, the Witch and the Wardrobe
In The Chronicles of Narnia: The Lion, the Witch and the Wardrobe, mermaids are depicted with brown-black hair, tan skin, and silvery fish tails with pink fins. They are seen near the end of the film swimming, leaping, diving, and laughing in merriment near the shores of Cair Paravel.

Pirates of the Caribbean: The Curse of the Black Pearl
When Elizabeth Swann warns James Norrington's royal marines of Hector Barbossa's cursed crew, Lieutenant Gillette jokingly says "Don't worry, Miss. He's already informed of that. A little mermaid flopped up on deck and told him the whole story."

Pirates of the Caribbean: On Stranger Tides
Beautiful, conniving, and dangerous, these mermaids will do anything to protect their home in Whitecap Bay. Whitecap Bay is the final leg of the journey for the Fountain of Youth. It is also where they have been known to gather for hundreds of years, thereby striking terror in the heart of sailors and pirates alike. Drawn to the surface by a singing sailor, they entrance their prey before dragging them to the depths of the bay to devour them.

The mermaids must defend against Blackbeard and his crew as they attempt to capture a live one to harvest her tear demanded by the Fountain of Youth ritual.

Once Upon a Time
Mermaids are a humanoid species featured on ABC's Once Upon a Time, notable for having the ability to swim across realms. They are native to the Enchanted Forest and Neverland, and first appear in the first episode of the third season. They are described as the most dangerous creatures in all the seas and are considered more of a threat than sharks, whales or even a kraken. Their powerful tails can break through wood and in large swarms they can sink ships. Mermaids can also use shells as trumpets to summon and manipulate terrible storms and waves. Some mermaids have voices which are lovely enough to enchant people and lure them onto rocks. So far, only three mermaids have been shown on show, one being Ariel, Ursula before becoming the Sea Witch and another unnamed one.

Trivia

 * In the Kingdom Hearts franchise, King Triton initially deduced that Sora was from another kingdom in the ocean due to his tail, implying that their tails act in a similar manner to human fingerprints (being unique).
 * Shortly after King Triton defeated the Sorceress in the episode "Red", he mentioned to the Sorceress "See you next century," implying that their lifespans are over one century (100 years), at the very least.
 * In real life, the classification of merpeople is debatable due to possessing fish-like traits (namely fish-tails). However, the presence of mammary glands implies that they are mammals. In addition, their tails are patterned in a horizontal manner, more akin to marine mammals than true fish, whose fish tails are patterned vertically.
 * A common claim was that merpeople were actually manatees or dugongs that were surfacing and had seaweed on top, causing them to be mistaken for them. The first part of the Disney Comics story "Serpent-Teen" alludes to this claim by having a manatee named Eddie pose as Ariel, implying that they have frequently used manatees as decoys to ensure they weren't discovered.
 * The TV series adaptation of the film Aladdin features a siren water elemental named Saleen, who is similar to a mermaid. However, her fish features resemble that of a lionfish, and she possesses the power to manipulate water.