Hakujaden (1958) IMDb
Review refers to an untranslated copy, for which my amateur Japanese did not entirely suffice.
In uchronian China, where costume influences stretch across about a thousand years of real history, a boy is forced to discard his pet snake because the adults disapprove. However, the snake returns in the guise of a beautiful “snake fairy” when they are both grown. A priest (?) believes the magical woman is demonic, causing much trouble.
This, “The Legend of the White Serpent”, is the first coloured cel-animated cinematic feature of Japan, hence the first anime in minor ways. It’s also the first of a series of similar films from Toei. It’s strongly influenced by Disney, but the mode of its adaptation owes as much to Die Nibelungen (1924) as to US appropriation.
The Disneyesque musical numbers are few and relatively simple, but the anthropomorphized cute-aesthetic animals are not. The fish turned servant is reminiscent of later imouto types. The amorality is distinctly non-Disney; the lord of hell is kind, and the villain is also the man who weds the happy couple. Incidentally, the Freudian potential is through the roof: Adults disapprove when a boy discovers and plays with his pale “snake”, and it continues to be a social problem as he ages and resumes the habit instead of forming a breeding pair.
References here: The Adventures of Hols, Prince of the Sun (1968).