Here's why:
* Magical Elements: While not overtly magical, the story features a transformation, which can be seen as a magical element. The duckling's change into a beautiful swan is a dramatic and unlikely event.
* Animal Characters: The story features animals as the main characters, which is common in fairy tales.
* Moral Lesson: The story carries a moral lesson about acceptance, self-worth, and the importance of inner beauty. Fairy tales often convey messages and life lessons.
* Archetypal Structure: The story follows a common fairytale structure of a protagonist facing challenges and ultimately achieving a happy ending.
While "The Ugly Duckling" also shares elements with fables (due to its moral message), it is primarily considered a fairy tale for its magical elements and overall structure.