Mermaid
From wipipedia.org
A mermaid is a mythical marine creature that has the appearance of a (naked) woman down to the waist and a fish's tail below the waist. The word comes from mer (sea) + maid. The male counterpart is a merman.
In mainstream fashion, dresses which are tight around the upper legs, but which expand out around the lower legs, are known as "mermaid dresses" from their resemblance to a mermaid's tail.