Merpeople are sentient beings who live underwater in lakes and seas, although they can breathe air for short periods of time.
Merpeople are intelligent folk who have developed their own culture and social structures and have a great love of music (FB, (GF26). Merpeople are found all over the world, including the Lake near Hogwarts (GF26) and Loch Lomond (FW). The voices of merpeople sound like screeching above water but are understandable beneath the waves, where they sound croaky. Their own language is called Mermish (GF26).
The merpeople of Hogwarts - who are selkies - have greyish skin and long, wild, dark green hair. Their eyes are yellow, as are their broken teeth, and they wear thick ropes of pebbles around their necks. Their powerful, silver fish tails allow them to swim expertly, while the upper parts of their bodies resemble those of humans. Merpeople are very tall. Harry encountered one in the lake near Hogwarts who was seven feet tall with a long green beard and a choker of shark fangs, carrying a spear.
At Hogwarts, the merpeople's village is located in the center of the lake, a quarter of a mile from the castle (GF26). The edge of the village is marked by a large rock with paintings of merpeople on it; they are carrying spears and chasing the giant squid. The village consists of clusters of crude stone dwellings stained with algae. In the center of the village, the houses line a village square in which rises a crude sort of statue of a gigantic merperson hewn from a boulder.
Some houses have gardens of weed around of them, and a few merpeople keep a pet grindylow tied to a stake outside their homes.
The merfolk who live in the lake near Hogwarts are led by Merchieftainess Murcus.
Apart from the grindylows kept by some of the Hogwarts merpeople, merpeople in general may keep various kinds of domesticated animals. In 1949, some merpeople caught a hippocampus off the coast of Scotland and domesticated it, for instance. Merpeople sometimes use lobalugs as weapons (FB).
Oddly enough, merpeople were not represented in the Fountain of Magical Brethren at the Ministry of Magic. This could not be simply because they have declined formal "being" status with the Ministry of Magic, because centaurs made a similar decision and were represented in the Fountain (OP7).
"Mere" is an Old English word meaning "sea," and "merpeople" is a gender-inclusive alternative to the more traditional moniker of "mermaid".
Merpeople around the world vary in appearance just as humans do. The sirens of Greece are the oldest recorded merpeople, who match the descriptions of Muggle legend (and the painting in the Prefects' bathroom), while the selkies of Scotland and the merrows of Ireland are depicted as less beautiful than the traditional folklore.
"Mermaids are sometimes associated with perilous events such as floods, storms, shipwrecks and drownings. In other folk traditions (or sometimes within the same tradition), they can be benevolent or beneficent, bestowing boons or falling in love with humans." (Wikipedia)