The Clefairy, in my view, are heavily based on fairy lore from many different cultures; small supernatural beings feature in Celtic, English, German, and Slavic tales. For instance, pre-Christian Irish religions spoke of the Tuatha de Danann, the elf-like "Fair Folk". Magical elves also figure prominently in Norse mythology--Valkyries might even be considered fairies themselves. Welsh traditions contain references to "gwyllions," little mountain faeries (though these are much less gentle than Clefairy!), and ancient Greek and Roman cultures had concepts of minor divinities known as "daimons", meaning "spirits."
In many of these myths and stories, these beings are generally friendly to humans, enjoy nighttime music and dancing, and leave no evidence of themselves in the morning besides a ring of mushrooms in the yard, known to this day in some regions as "fairy rings." Additionally, these creatures can use magic (often very powerful magic), and their activities are often tied to moonlight or to the lunar cycle in general. (Sounding familiar yet? LOL)
As a youngster growing up watching Unsolved Mysteries and also reading about ghosts, aliens, and fairies, I've always had an interest in supernatural happenings and beings. So I suppose it's pretty much natural that Clefairy is my favorite Pokémon. Even though I know that stories of the supernatural aren't necessarily real, it's fun to imagine that there's a whole other world of activity going on right beneath our human noses!