What is mermaid?

You can find the character "Mermaid" in folklore, it's an aquatic creature with the head & upper body of a female human and the tail of a fish. It appears in the folklore of many cultures in the world (Near East, Europe, Asia, and Africa). 

Here are some paintings of the mermaid those are related to cultures:

Ilya Repin, Sadko in the Underwater Kingdom (1876).

Suvannamaccha and Hanuman, mural at Wat Phra Kaew, Bangkok.

Arthur Rackham, Rhinemaidens, from The Rhinegold & The Valkyrie (1910).

John William Waterhouse, A Mermaid (1900).

The Fisherman and the Syren, by Frederic Leighton, c. 1856–1858

Raymond discovers Melusine in her bath. Jean d'Arras, Le livre de Mélusine, 1478.