Merfolk

Mermaids are water-based creatures traditionally imagined as having a top half indistinguisable from humans and a fish-like lower half, usually in the form of one tail with caudal fin at the end. There are many variations, though, which are largely of recent nature. Mermaids are predominantly thought of as female and male ones tend to be given the designation of "mermen", or the creatures are degendered with terms like "merfolk" or "merperson". The nature attributed to mermaids ranges from murderous, for either entertainment or food, to saviorhood of children and sailors.

Etymology
The modern word "mermaid" has its roots in the first half of the 14th century as "mermayde", prior to which the creature was known as a merewif or meremenn. The recurring "mer" portion means "sea" or more generally "large body of water", and words like it can be found in many European countries. The suggested Proto-Indo-European word is therefore "móri".

Mermaids are sometimes conflated with sirens, such that some European languages do not differentiate between the two.

Stories featuring seafolk have started playing with mermaid variations as to the shape and qualities of the tail since the second half of the 20th century. Of these, the only one with a name of its own are the octopus-based seafolk, which are named "cecaelias" or "cilophytes". These names originate from a story featuring such a creature named "Cilia" in issue #16 of the Vampirella comic magazine, released in April of 1972. The creature is named Cilia and introduces herself as a cilophyte. "Cecealia" is a corruption of "cilia" and as such has no meaning, while it is unclear what "cilophyte" is supposed to mean. "-Phyte" comes from Ancient Greek "φύειν" ("phuein", "to grow"/"to generate") through "φυτόν" ("phuton", "plant"/"growth"/"descendent") and probably means "growth" in context. The "cilo-" bit is a mystery, though one theory is that it's an adaption of "Scylla" (which in English is pronounced with a silent "c"). The Scylla is an Ancient Greek monster who's sometimes depicted as having multiple tails. The two-tailed mermaid, for instance, is based on Etruscan images of her. As such, it's possible the author aimed for a name essentially meaning "growth like Scylla".

History
In 1989, The Walt Disney Company released The Little Mermaid, an adaption of the Andersen tale. It placed the Ancient Greek Triton, complete with trident and golden palace, in the role of King of the Sea and father of the titular mermaid, who is named Ariel in Disney's work. Many details were altered to make the movie kinder than the fairytale, such as all the parts relating to souls and the bittersweet ending. One choice with lasting influence is the design of the witch, who is a regular mermaid in the original tale but one with an octopus-like lower body. For the movie, she is the only 'alternative mermaid' present and creatures like her are regularly referred to as sea witches since. However, Disney greatly expanded the movie's underwater diversity in additional The Little Mermaid media. The witch was given a sister in 2000 as the sequel to the 1989 movie needed a villain of its own, but prior a small army of octopus-based seafolk appeared in the 1992-1994 TV series, specifically in "Heroes" of Season 3. They are identified as octopans. The series also introduced manta ray-based seafolk in the form of Evil Manta and his son. Their species wasn't given a name, but with four episodes across all three seasons, Evil Manta was a regular presence. Lastly, a series of four The Little Mermaid comics were published in 1992. The first two, brought out in February and March, together hold one story, "Serpent-Teen". This story features eel-based seafolk, known as the moray, named after moray eels despite having the electrogenic abilities of electric eels. All of the alternative mermaids of Disney's The Little Mermaid are designed with full-body non-human skin colors, unlike the mermaids, and they are also all enemies of the mermaids. Nonetheless, Disney's The Little Mermaid is the biggest franchise to feature alternative mermaids.

Monster High
The Monster High cast of mermaids is limited to Sirena Von Boo and her mother as well as Rider. There's also a Create-A-Monster set shaped like a mermaid, but marketed as a siren.