Merpeople

Merpeople are legendary creatures that are half human and half fish. They are featured most heavily in The Little Mermaid and other related media. Female merpeople are referred to as mermaids, while males are referred to as Mermen. Other than The Little Mermaid Franchise, Merpeople have made appearances in films such as the Peter Pan films, Jake and the Never Land Pirates, Gravity Falls, and The Thirteenth Year.

In 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. Merpeople are shown to coexist with sea life, with whom they are able to communicate. Merpeople generally do not interact with humans, due to the danger, though this changes after the events of the first film. The first film also shows that many unnamed merpeople are held as prisoners of Ursula. They presumably made bargains for her aid, and were unable to uphold their part of the bargain. As a result, they are withered down to polyps. Ursula's death causes them to be restored to their natural forms.

Appearancewise, they are human from the waist up. The lower body consists of a long fish tail, which can be of any color. 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 the character refers to them as "half-fish, half-something else". Mermen generally appear barechested, but females wear a bikini top of some fashion. It is also possible that Merpeople are Herbivores, or at the very least are Vegetarians, as they aren't seen eating Fish, and apparently they don't seem to view eating fish as being ethical.

Abilities
Merpeople have the ability to metabolize Oxygen from the water, which negate the need to surface to breathe air. However, Merpeople are more than capable of breathing air outside of the water, as evidenced by certain events where Merpeople are on land for prolonged periods of time.

Also, Merpeople are capable of jumping out of water in a similar fashion to Dolphins as well as swimming at such a speed as to rival most vessels during that time. Merpeople also have a degree of Superhuman Strength, which is evidenced by Ariel moving an underwater boulder from the grotto as well as knocking down a door with relative ease. Similarly, they also seem to have a form of Superhuman endurance, able to survive a fall into a whirlpool that is about several hundred feet deep, as evidenced by Ariel in the final fight with Ursula. They are also more than capable of speaking with aquatic life, or at the very least marine life period. Merpeople are also capable of practicing Magic, as evidenced by King Triton's Trident as well as Ariel practicing magic at one point.

Outside of The Little Mermaid franchise
Other merpeople feature in the film, Peter Pan, Jake and the Never Land Pirates, Gravity Falls, as well as in the Disney Channel Original Movie, The Thirteenth Year.

Peter Pan
In Peter Pan, which was made before The Little Mermaid, the mermaids are similar in appearance to their later counterparts. They live in Mermaid Lagoon in Never Land, 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 by the name of Marina helps Jake, Cubby, Izzy, and Skully in some episodes, also Stormy is seen in some of the episodes Marina is in.

The 13th Year
In The Thirteenth Year, two merpeople are shown: the protagonist, Cody, and his biological mother. Merpeople in this film are somewhat similar in appearance to merpeople from other media. However, there are several distinctions. In this film, merpeople 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. Other than the abilities listed above in the TLM franchise, Merpeople also seem to possess other abilities, such as the ability to generate electricity, to the point that they can fire it similarly to a defibrillator, and the ability to stick to surfaces.

Gravity Falls
Merpeople were briefly mentioned in "Dipper vs. Manliness" by the Manotaurs, and one 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 Mystery Shack gift shop, although it resembles the famous hoax Fiji Mermaid. In the opening credits, a picture of a merperson 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" as Mabel befriends a beautiful one 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 Mabel Pines saw him for the first time, she went all silly, wanting him. Mabel soon realizes that he is a merman, and thus tries to help him, of which she succeeds. Mermando is last seen swimming back to his home. Merpeople have about 17 hearts.

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 merpeople's tailfins and 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 Merpeople's 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. However, the presence of mammary glands implies that Merpeople are mammals.