Muggles' Guide to Harry Potter/Magic/Merpeople

Overview
The Merpeople are a race of aquatic people, at least one group of whom live in the lake on the grounds of Hogwarts.

Extended Description
Merpeople do not much resemble the Muggle idea of mermaids and mermen. They are very thin, with very long arms and a fish's tail; some are armed with spears. They have high-pitched voices, and seem to be able to speak, or at least parrot, some English words; however, among themselves they generally speak their own language, Mermish. Above water their singing or speech sounds like a screech, yet underwater it is eerie and hypnotic.

In Harry Potter and the Goblet of Fire, to solve the puzzle of the second challenge, Harry makes use of the Prefect's bathroom. There is a painting of a mermaid on the wall there; however, as Harry discovers when the Challenge actually happens, the painting is very different from the reality. The clue is actually a merpeople song, that has to be listened to under water to be understood.

The second Challenge actually is to rescue four people, one for each Champion, from under the lake. They are guarded by Merpeople, and the way the Challenge is worded, it would appear that they had kidnapped the people; in fact, however, the four "detainees" (for want of a better word) were Stunned by Professor McGonagall and handed over to the merpeople.

Before points are awarded for completion of the challenge, Professor Dumbledore consults with the mer-chieftainess to determine what actually happened; the fact that Harry had waited and then rescued detainees other than the one that was his main concern, as confirmed by the mer-chieftainess, counted in his favour in the competition.

In Harry Potter and the Half-Blood Prince, the Merpeople come near the surface of the lake and sing a haunting song at Dumbledore's funeral.