KINGSLAYERS mermaid lore
in the words of my good friend @iolitemoth, "Hand It Over".
Anyway, there are in fact many humanoid critters creaturing about in the world, but the only ones I've really worked out so far are the mermaids, though technically the plural form of the word in the world is merfolk/sirens (both are used interchangeably).
Their appearance is typically somewhere between DIsney Meramids, Greek Sirens, accurate scary fish-like mermaids, the Deep Ones from The Shadow Over Innsmouth, that fish guy from that weird Sally Hawkins movie, and carnivorous fish.
Here is a helpful chart I made.
At first the merfolk were just kinda vibin in the oiter, and sometimes would peek their heads out of the oiter to peep the dry land. As humanity begin to progress in like society and technology and language and stuff, the merfolk took notice and started telling each other about it, and even copying what the 'landfolk' were doing. They also tried speaking the languages humans spoke, and over may millenia the continous efforts of the mermaids over generations eventually changed their vocal cord structure in a way that allowed them to speak human languages. Though when they speak in those languages they tend to have raspy, garbled voices that can sometimes be hard to understand.
(The mermaids actually have their own language that is still used today, but it's very hard for humans to speak. They learned from mermaids what the words MEAN, but it's just very hard for them to say the words.)
The humans knew of the existence of merfolk of course. At first, they had no idea what to do about them. All they knew was that sometimes around ports/beaches they could catch some fishy losers peeping their heads out and staring at them with them big ol eyes, and babble at them. Occassionally they would unintentionally steal something that was left on beaches or ports/dropped into the water by accident, though they were normally returned once the merfolk were done rotating it around in their hands.
In some areas at the time, mermaids were actually hunted as a food source/for sport because "well theyre basically fish lol" but nowadays thats considered just murder.
Anyway, over time, more and more people (namely Big Brain people but occasionally your average John) started trying to communicate with the mermaids, and eventually could actually talk with them and learn more about their cultures. There are also documented cases of how as far back as ancient Greece, merfolk were basically just allowed around seaports to help remove barnacles from ships and recover lost items, and a story of a pirate crew who accepted a merman as a crew member.
Nowadays, merfolk and landfolk co-exist harmoniously. There are actually some coastal cities/towns/whatevers that have been built specifically to allow landfolk and merfolk to like. literally live together. The most famous of these cities are Miami, Tokyo, Athens, Rio de Janeiro, Sydney, Algiers, and Negombo. The merfolk do actually have their own underwater settlements, and while they aren't as advance as building on the land, they are still very impressive in their own right.
There's also a study built around understanding the cultures of merfolk more, known as Hydro-Anthropology (also Marine Anthropology and Limno-Anthropology, as specific terms for freshwater/saltwater mermaids.) On the flip-side, mermaids have Geo-Anthropology, which is the study of landfolk culture.
Merfolk have wildly varying appearances based on their environments. Sometimes they have a set of legs along with arms to maneuver better, sometimes they have fins instead of arms, sometimes they have more than two eyes, sometimes they have suckers on their palms, forearms, and tails for climbing along rocky walls, and sometimes they even have claws. Their appearance also changes based on the area of the world they live in, or how deep in the ocean they live, and how some mermaids have been found in very weird areas, which I will plan on talking about in another post because it's a very interesting topic to me.
Sirens can also use magic like landfolk, but the majority are water and plant mages, with the occasional earth mage. For a siren to have something like, say, fire magic, they would need to have a family with a fire mage.
Last thing, merfolk and humans have actually been known to have children together, called land-sirens. They can look anywhere from humans with mildly colourful skin to just a mermaid with human eyes. Typical traits seen among most land-sirens is pale skin of unusual colours, gills, sharper teeth, naturally raspy voices, fish tails, and occasionally webbed fingers/toes + fins. They can also breathe both in water and on land. The earliest documented cases of a land-siren came from 1387 somewhere in the Holy Roman Empire, though some art from the 1100s may possibly depict a land-siren.
oh and theres also no comedic instances of "mermaid is in shock because they saw someone eat fish??!?!?!?!?!" because the mermaids eat fish as well lol
Alright thats enough mermaid lore for now I'll probably expand on it more soon