Naiad

The naiad is an elemental water spirit, serving as the embodiments of rivers, lakes or fountains.

Source
Naiads originate from greek mythology.

Media
Naiads appear in the following media:

Games
Lamia (in The Witcher) are similar creatures, although described as half woman and half snake.
 * Titan Quest
 * The Witcher - those Water nymphs or Vila are beautiful female creatures, whom derive from slavonic mythology. Thoughout the series, there are many different water nymphs, but the Lady of the Lake probably is the most impressive and powerful. It's not proven, if she is just a local goddess or maybe more. While those Water Nymphs live near lakes, rivers or ponds, they still have legs and no fish tail. In The Witcher III - Wild Hunt, the Witcher also encounters various hostile Sirens, which not only have a fish tail (while looking like women from head down til the belly button), but also wings. Notable is Sh'eenaz, a mermaid from the Witcher Short Story "A Little Sacrifice" from the Sword of Destiny collection by Andrzej Sapkowski.
 * The Witcher - those Water nymphs or Vila are beautiful female creatures, whom derive from slavonic mythology. Thoughout the series, there are many different water nymphs, but the Lady of the Lake probably is the most impressive and powerful. It's not proven, if she is just a local goddess or maybe more. While those Water Nymphs live near lakes, rivers or ponds, they still have legs and no fish tail. In The Witcher III - Wild Hunt, the Witcher also encounters various hostile Sirens, which not only have a fish tail (while looking like women from head down til the belly button), but also wings. Notable is Sh'eenaz, a mermaid from the Witcher Short Story "A Little Sacrifice" from the Sword of Destiny collection by Andrzej Sapkowski.

Novels

 * The Moon-Bog H.P. Lovecraft (1921)