You might think I was out of things to say about pillywiggins, but you would be wrong! I've begun keeping a list of any books I can find that mention them. This is an opportunity to see how new folklore develops in the era of the Internet. There are far too many works to list here, but here are a few:

​Haunts & taunts: a book for Halloweén and all the nights of the year by Jean Chapman (1983)
At this point, the second book ever that I know features pillywiggins. Here, "Pillywiggins" is the given name of a baby fairy in a retelling of the fairytale "Katie Crackernuts." The name also appears in a list of fairies later on.

Fairies & Elves, the "Enchanted World" series (1985)
The Enchanted World was a twenty-one-book series released by Time Life Books. The books were edited by Ellen Phillips, with Tristram Potter Coffin as primary consultant. The series was available through mail order. TV commercials struck a creepy, mysterious tone and featured horror actor Vincent Price. Fairies & Elves was book three. It primarily recounts folktales from around the world, with one very brief description of pillywiggins, mentioning that they hide inside flowers and are about the size of bees. 
The illustration shows long-haired, butterfly-winged nymphs peeking out of the tops of wildflowers. This is the earliest picture of pillywiggins I have found, unless you count the unidentified dancing gnomes in Haunts & Taunts. 

Various books by Pierre Dubois, 1991-onwards
Dubois has mentioned pillywiggins in many of his works, but I am unsure what the first one was. I know he was including them at least by 1991, namedropping them in his graphic novel Pixies. 
The thing about Dubois is that he is a creative writer, not a folklore scholar. He regularly alters folklore creatures to suit his purposes. Parisette (a plant) and Tisanière (an herbal tea infusion) show up as fae creatures in his books. “Freddy” – Freddy Krueger! - gets an entry in a list of bogeymen. And then there are the many creatures which appear nowhere before Dubois's writings: Danthienne, H'awouahoua, Lorialet, Scarille, Tiddyfollicoles, etc. Many of these have since made their way into other fantasy works.

In La grande encyclopédie des lutins (1992), Dubois claims to have found a mention of pillywiggins among other fae in an 11th-century manuscript titled Aelfsidem, translated by "W.T. Dodgsons Luchtat, 1334, Meinster, p. 526." Dubois and people quoting him are the only ones ever to mention this manuscript. In addition, the "quotes" from Aelfsidem read exactly like everything else Dubois writes, and the fairies, pixies, and undines it lists are highly anachronistic. (If you look for fairylike beings in medieval manuscripts, you’re more likely to find incubi, neptuni, fauni, and dryades.) Dubois frequently makes up fictional quotes in the playful way of  a fantasy writer building a world. One example is his famous scholar “Petrus Barbygère” - who is, in fact, a fictional character and the lead of one of Dubois's comic series. This has not stopped a few confused authors from quoting Dubois's in-universe books and people as sources.

His Encyclopédie des Fées gives a longer description of pillywiggins, explaining that they have insectoid characteristics and can take the form of bees or dragonflies. The text is rife with errors. Francis James Child becomes "Frances Jammes," and the artist Cicely Mary Barker is "Cecily Mary Broker." Nevertheless, Dubois' work ushered pillywiggins into French fairylore.

A Witch’s Guide to Faery Folk by Edain McCoy (1994) 
Okay, so first off, McCoy has been a figure of some controversy among Wiccans, being particularly infamous for claiming that the potato was sacred to the ancient Irish Celts. (The potato was introduced to Ireland around the 16th century.)

She bases this book in folklore, but then runs in her own direction with it, creating fanciful and detailed descriptions for various “faery” races. She introduces the "saleerandee," a Welsh lizard faery whose name resembles the salamander, and the “attorcroppe,” a serpent-like faery. She is the only source for these creatures. She also transforms the Yucatec deity Sip into teensy, shy Mayan fairies called Zips and the German moss-covered dwarves, Mooseleute, into the pretty butterfly-winged Moss People.
She makes the pillywiggins friendly, cute and sweet, concerned only with blossoms and springtime. They ride upon bees. She names their sexy, scantily-clad blonde queen "Ariel." Everything about this Ariel corresponds to the Ariel of Shakespeare's Tempest.

• Sings
• Rides on a bat
• Sleeps in a cowslip
• Associated with bees
• Associated with flowers (cowslips)
• Associated with summer/warm seasons
• Controls the wind (Ariel stirs up a tempest)
• No longer seen, but still exists (Ariel is freed at the end)

But Shakespeare's Ariel is male, you say! Well, androgynous, anyway. Ariel is not not a buxom blonde in a white nightie.
In fact, in art and costuming, Ariel is frequently a feminine figure with golden hair and gauzy white clothing. 

McCoy may have been inspired by Fairies & Elves’ mention of cowslips and honeybees, combined with Ariel’s speech in The Tempest. There would have been plenty of available artwork featuring Ariel that increased the similarities.

McCoy’s guide to fairies has been copied and circulated online in its entirety since at least 2001.

A Basket of Wishes, by Rebecca Paisley (1995)
A romance. Splendor, princess of the Pillywiggins, who has vast magic powers and weeps diamonds, has to bear some human guy's child.

Here, Pillywiggin is a realm of Faery. Its inhabitants are referred to as Pillywiggins, Pillywiggin fairies, pixies, sprites, elves or imps. Their natural form is tiny humanoids the size of "the span of a large butterfly's wings." They can grant wishes as well as fly with or without the aid of wings (which are apparently detachable). Physically, they are identical to humans except that they are incredibly beautiful and exude stardust, not unlike Tinker Bell. Overall, they serve as an amalgam of fairy traditions old and new. Their queen is the Tooth Fairy.

Buttercup Baby by Karen Fox (October 2001)
A romance. Ariel, queen of the Pillywiggins, who has vast magic powers and weeps opals, has to bear some human guy's child.

Karen Fox wrote four books for Jove Books' "Magical Love" series, and she used pillywiggins as a race of garden fairies. They apparently make up the majority of the fae and serve King Oberon. They seem uniformly female and very beautiful, and they are distinct from pixies (mischievous miniature trolls who serve Queen Titania). Fox's use of Queen Ariel points to Edain McCoy's work.

Pillywiggins and the Tree Witch, by Julia Jarman (2011)
A chapter book for younger readers, in which Pillywiggins is the personal name of one fairy.
Jarman's Pillywiggins is a refreshing departure from the twee miniature flower goddesses seen elsewhere on this list. She's a tough, tomboyish loner, repeatedly contrasted with the other sparkly pink fairies. Although she looks eerie, she's a heroic figure. There is a scene where she senses plants coming alive on Midsummer’s Eve, but otherwise she doesn’t seem to be associated with flowers that much. 
I contacted Ms. Jarman and learned that this Pillywiggins was based on a doll found at a craft fair. Ms. Jarman turned to the Internet in order to find out what the name meant, which is the case for many authors and researchers these days, including me.

Atlantide: La naissance by T. A. Barron (2016)
This is a weird example. In the original English version, Atlantis Rising, the characters encounter tiny forest fairies with wings and antennae. These fairies are called "Quiggleypottles." In the French translation, their name is mysteriously replaced with "pillywiggins."
I’m not sure why it was necessary to substitute anything for Barron’s original creation. Does Quiggleypottle sound bad in French? This does indicate, however, that the translators were familiar with pillywiggin as a word for tiny flower fairies. 

CONCLUSION

Pillywiggins have inspired songs (at least three; the ones I've found are either French or attribute pillywiggins to French tradition). They are monster-insects in the video game Final Fantasy XI. Quite a few small businesses are named after them. You can buy pillywiggin dolls on Etsy. A book of cat names suggests Pillywiggin as a charming name for a kitten. And, as of April 2019, a pillywiggin has appeared in the anime "Fairy Gone" (though it doesn't look much like a traditional fairy).

In 1697, Charles Perrault published the story of "Cendrillon: ou la Petite Pantoufle de verre" (Cinderella, or the Little Glass Slipper). This is probably the most widespread version of Cinderella, thanks in large part to its adaptation by Walt Disney. I often see people on the Internet insist that "the original Cinderella wore gold slippers and had the stepsisters cut off their toes and get their eyes pecked out by birds!" But that was "Aschenputtel," the version from the Brothers Grimm. Perrault published his work 115 years before the Grimms did. It's impossible to identify an "original" version of Cinderella, but at least in terms of publication, the glass slipper came first.

But was it really a glass slipper? There is a persistent theory that the shoes were originally made of fur - which is about as far from glass as you can get! 

This theory may have originated with Honore de Balzac, in La Comédie humaine: Sur Catherine de Médicis, published between 1830 and 1842 and finalized in 1846. The word for glass, "verre," sounds the same as "vair" or squirrel fur. This fur was a luxury item which only the upper class was allowed to wear. Therefore, claimed Balzac, Cinderella's slipper was "no doubt" made of fur. 

Since then, quite a few authors have relied on this alternate origin for Cinderella's origins, usually in order to fit the story into a more "realistic" mold. It has also produced a persistent legend in the English-speaking world that Perrault used fur slippers and was mistranslated.

Yes, glass shoes raise questions. How did she dance in them? How did she run in them? Wouldn't they have shattered? Wouldn't they have been super noisy? Fur slippers erase those questions entirely. 

But Cinderella stories regularly include things like dresses made of sunlight, moonlight and starlight. Forests grow of silver, gold and diamonds. Prisoners are confined atop glass mountains. In Perrault's version alone, mice are transformed into horses and pumpkins into carriages. 

Glass slippers should not be an issue.

In fact, they fit perfectly well with the internal logic of the fairytale. As has been pointed out by others, the whole point of the slippers is that only Cinderella can wear them. Fur slippers are soft and yielding. Glass slippers are rigid and you can see clearly whether they fit a certain foot. Moving into symbolism: they are expensive, delicate, unique, magical. Cinderella must be light and delicate, too, in order to dance in them. They are a contradiction in terms (of course it would be impossible for a woman to dance in glass shoes! That's the whole point!) and that's why they have captured so many imaginations. 

The fact is that Perrault wrote about "pantoufles de verre," glass slippers. He used those words multiple times. There is no question that he was talking about glass. No one mistranslated Perrault.

However, did he misunderstand an oral tale which mentioned slippers of vair? 

It's important to note that "vair" was popular in the Middle Ages. By Perrault's time, this medieval word was long out of use!

It is still possible that Perrault could have heard a version with vair slippers - but is it probable? What stories might Perrault have heard?

In her extensive work on Cinderella, Marian Roalfe Cox found only six versions with glass shoes. She found many that were not described, many that were small or tiny, and many that were silver, silk, covered in jewels or pearls, or embroidered with gold. A Venetian story had diamond shoes, and an Irish tale had blue glass shoes. Cox believed that other versions with glass slippers were based on Perrault's Cendrillon. Paul Delarue, on the other hand, thought these versions were too far away in origin, which would make them independent sources - which means Perrault could have drawn on an older tradition of Cinderella in glass shoes.

Gold shoes are perhaps significant. Ye Xian or Yeh-hsien, a Chinese tale, was first published about 850, and its heroine's shoes are gold. Centuries later, the Grimms' Aschenputtel takes off her heavy wooden clogs to wear slippers “embroidered with silk and silver,” but her final slippers - the ones which identify her - are simply “pure gold.” It’s not clear whether this means gold fabric or solid metal.

As for other early Cinderellas: Madame D'Aulnoy published her story "Finette Cendron" (Cunning Cinders) in 1697, the same year as Perrault's Cendrillon. Her heroine wears red velvet slippers braided with pearls. Realistic enough. 

The Pentamerone (1634) has "La Gatta Cenerenterola" (Cat Cinderella). It's not said what the heroine's shoes are made of, but she does ride in a golden coach.

Note that shoes are not always the object that identifies the heroine. In many tales, it's a ring - something likely to be made of gold or studded with gems.  What if, at some pivotal point, far back in history, a storyteller combined the tiny shoe and the golden ring into a single object? 

On the other hand, I have never found a Cinderella who wears fur slippers to a ball. Fur clothing appears in Cinderella stories such as "All-Kinds-of-Fur," but it's used as a hideous disguise. Rebecca-Anne do Rozario points out that "Finette Cendron" (which, again, came out the same year as Perrault's "Cendrillon") has Finette instruct an ogress to cast off her unfashionable bear-pelts. Fur clothing was not a symbol of wealth or status, but of wildness and ugliness.

Glass slippers were most likely Perrault's own invention dating from when he retold his folktales in literary format.  No translation error, no misheard "vair" - just a really good idea and his own storytelling touch. If anything, he probably heard stories where the slippers were made of gold, or where their material was not mentioned. 

It was only later writers like Honore de Balzac who added the confusion of the squirrel-fur slippers, and folklorists and linguists have been arguing against it ever since. James Planché wrote in 1858, "I thank the stars that I have not been able to discover any foundation for this alarming report." That was twelve years after Balzac's book was officially completed.
Heidi Anne Heiner at SurLaLune points out that the vair slipper theory dismisses Perrault's "adept literacy," and "negates [his] interest in the fantastic and magical, discounting his brilliant creativity."
Unfortunately, as shown by Alan Dundes, the vair/verre theory made it into influential sources such as the Encyclopaedia Britannica, and thus has been fed as fact to successive generations of readers. Who's going to question the Encyclopaedia Britannica? And so this rumor remains persistent.

As a final bit of trivia: there is one mention of glass shoes in the Brothers Grimm's tales. “Okerlo” appeared only in their 1812 manuscript and was quickly removed. (This is perhaps because it is clearly a retelling of a French literary tale, “The Bee and the Orange Tree.” Not unusual for the Grimms, but in this case it may have been just too blatant.) In the final lines, the narrator is asked what they wore to a wedding. They describe ridiculous clothes, with hair made of butter that melts, a dress of cobwebs that tears, and finally: “My slippers were made of glass, and as I stepped on a stone, they broke in two.” Here, the destruction of the fairytale clothing points out how impossible the magical tale is, and signals the end of the story and a return to reality. ​

Sources

• Cox, Marian Roalfe. Cinderella: Three Hundred and Forty-five Variants of Cinderella, Catskin, and Cap O'Rushes. 1893. especially pp. 506-507.
• Do Rozario, Rebecca-Anne. Fashion in the Fairy Tale Tradition: What Cinderella Wore. 2018. pp. 203-204.
• Dundes, Alan, ed. Cinderella: a Casebook. 1988, pp. 111 (article by Delarue)
  Heiner, Heidi Anne. History of Cinderella. SurLaLune.
• Language Log. CINDERELLA'S SLIPPERS: GLASS OR SQUIRREL FUR? February 28, 2006.
• Mikkelson, David. "Glass Slippers Mistranslations: Were Cinderella's slippers made of fur, not glass?" Snopes. 
• Examples of ATU 510A, Cinderella
• Examples of ATU 510B, The Father Who Wanted to Marry His Daughter