While The Little Mermaid contains common fairy tale and folklore elements, especially those about mermaids, this fairy tale is the literary creation of Hans Christian Andersen. The tale has no direct oral predecessor in folklore.
The closest percursor of the tale is Undine, a story by Friedrich de La Motte Fouque. Andersen admits to his familiarity and consideration of Undine while writing his Little Mermaid. In many ways, his tale is a direct reaction to the earlier story.