The epic media franchise that is Doraemon has been going strong since the 1980s, and in 2010 they added a new movie to the series: Doraemon the Legend 2010, or Doraemon the Movie: Nobita's Great Battle of the Mermaid King.
In this movie, Nobita, Doraemon and friends encounter a mermaid princess named Sophia. She is the heiress of the mermaid tribe, a technologically-advanced race of people that came to Earth thousands of years ago from the planet Aqua, and were initially worshipped as gods. The mermaid tribe escaped Aqua because the planet was dying thanks to the greed of the rival merman tribe, but now the new leader of the merman tribe, Bulkin, has tracked them down. Bulkin wants the fabled mermaid sword that he believes he can use the conquer the galaxy. It's up to Sophia and her new friends Nobita, Doraemon, Shizuka, Dorami, Gian and Suneo to save the day. It's a pretty epic adventure story with a touch of space opera.
The early portions of the movie follow Sophia's accidental arrival on land and how she gets to know Nobita and friends while exploring the human realm. They don't know she's a mermaid at first because she has legs, though they later discover Sophia's true identity upon learning that her clothes can transform her legs into a tail when she wants to swim underwater. The mermaid tribe itself is a thriving alternate society that exists deep in the ocean, and the second half of the film features the gang getting tails-transforming clothes of their own to visit Sophia's home.