Once upon a time is an example of a inverted fairy tale.
Once upon a time, meanint at some time in the past is usually used as a conventional opening of a story.
A fairy tale is a folklore genre in the form of a short story. Such stories generally depict invented creatures such as dwarfs, dragons, elves, fairies, giants, gnomes, goblins, griffins, mermaids, talking animals, trolls, unicorns, or witches, and usually magic or enchantments.