What do you call a mermaid with legs?

A mermaid with legs is often referred to as a "human" or "land-dwelling version" of a mermaid. In folklore and fiction, when a mermaid transforms into a human, she may be called a "siren" or simply a "former mermaid." The concept varies across different cultures and stories, but the idea typically involves a creature that has transitioned from its aquatic form to a terrestrial one.