Merpeople

Merpeople are sentient beings (or beasts, as they want nothing to do with the likes of vampires and hags) that live underwater, and are found all over the world. Their customs and habits are mysterious, and, like the Centaurs, they refused Being status in favour of Beast. Merpeople can breathe above the waves for a time, but it is unclear if they can ever truly leave their habitat.

Merpeople society is based around organised communities, and they have a great love of music. Their language is known as Mermish, and only a few Wizards know how to speak it. However, since their language can only be heard properly underwater, it sounds like a loud screeching when spoken above water.

Merpeople have been known to keep Grindylows as pets, and use Lobalugs as weapons. One tribe even managed to capture and domesticate a Hippocampus.

Dolores Umbridge once requested the Ministry to have Merpeople rounded up and tagged, due to her hatred to half-breeds, though Merpeople are in fact a separate species of their own, not a mixed species between fish and human. Regardless of Umbridge's misunderstandings and hatred, this suggestion was never put into practice.

Sub-species
The term "Merpeople" is an umbrella term. There are several distinct sub-species around the world. Sirens  of Greece are the earliest recorded Merpeople, and are often depicted in Muggle literature and paintings as beautiful women who sing to lure sailors to their deaths. The Selkies of Scotland, and the Merrows of Ireland, are less pleasing to the eye. However, despite their physical differences, they all retain their love of music.

Behind the scenes

 * According to W.O.M.B.A.T., it is possible that freshwater merpeople are less warlike than salt-water merpeople.
 * In Lego Harry Potter: Years 1-4, the merpeople actually attack, whereas in the film and book they only confronted Harry when he tried to take more then his own hostage, but even then they only threatened to attack.
 * Mirabella Plunkett transfigured herself into a haddock after she was forbidden to marry a merman whom she was in love with.

Appearances

 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (video game)
 * Fantastic Beasts and Where to Find Them
 * Lego Harry Potter: Years 1-4
 * Harry Potter LEGO Sets

Notes and references
Êtres de l'eau Водяной народ Vedenväki Meermens