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.
Appearance wise, 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 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.
Merpeople have the ability to metabolize oxygen from the water, which negate 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, they 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. They 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. However, the latter ability, at least in Ariel's Beginning, may have its limits, as Athena was killed by being crushed by a shipwrecked vessel. They are also more than capable of speaking with aquatic life, or at the very least marine life period. They are also capable of practicing magic, as evidenced by King Triton's trident as well as Ariel practicing magic at one point.
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 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 one named Marina helps Jake, Cubby, Izzy, and Skully in some episodes. Also 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.
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. Other than the abilities listed above in the The Little Mermaid franchise, 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
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.
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 and likely gills). 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.