Hayao Miyazaki has said Ponyo began as his version of Andersen's The Little Mermaid, published in the 1837 collection: a sea creature who wants to become human for the love of a boy. Miyazaki gave it the happy ending Andersen did not.
Short Story· 1837· Fantasy
Den lille Havfrue (The Little Mermaid)
Den lille Havfrue (The Little Mermaid): 2 recorded works trace back to it.
What Den lille Havfrue (The Little Mermaid) influenced
Works that record this one as a source, with the evidence a contributor gave.
Andersen's tale gives the film the bargain with the sea witch, the voice as the price, and the legs that are agony to walk on. The ending is inverted: Andersen's mermaid is not chosen, cannot bring herself to kill the prince, and dissolves into sea foam, while the film marries her off. The change is the most discussed single decision in the Disney canon.