Ivan Bilibin’s Frog Tsarevna (1900). Prince Ivan burns her skin, so it takes an extra episode to get them to happily ever after.
When they arrived at the Fairy Tale Lobby, Flossie Squashblossom was holding forth.
“What I wanna know is, why a hedgehog?” Seals? Okay, I get it. Snakes? Sure, why not. Birds? Of course. They all symbolize so many human character traits — both flaws and strengths. But hedgehogs?
Sagacia looked puzzled. “How did they know the Fairy Tale Question of the Month when we hadn’t even gotten here to post it yet?” she whispered.
“I dunno,” Simplia mumbled ponderously. “Knowing Flossie, it could be a perfect coincidence! Sometimes she says weird things right out of the blue. ”
“Yeah.” Sagacia acknowledged. “Like a kid being born a hedgehog. What is that? Some kind of chipmunk?”
“I think so,” Simplia said. “Or a porcupine. You know: kind of prickly.”
Flossie overheard the last bit. “Could be,” she said, certain the response was to her own question. “But, in fact, he was a perfect gentleman. Not prickly at all, socially speaking.”
Everyone turned to look at the recent arrivals.
“We just came to post the FT?oM,” Sagacia smiled.
“Don’t let us interrupt,” Simplia added.
The small assembly did not.
“There’s a Tibetan fairy tale about a frog child,” said a voice from a dark corner.
“And Tom Thumb or Thumbelina, all born of the mother’s wish for a child, no matter what,” echoed another.
“But does the actual species matter?” the first voice asked. “I mean, that’s not the point of any of those stories. The point is that an effort is made to grant a wish. Sometimes it’s an I’ll teach you not to ask for something you’re not entitled to! and sometimes it might be an honest effort on the part of a magical wish granter to comply as nearly as possible.”
Charles Kiernan had been standing at the hearth. He tapped his pipe.
I’m going to get a little technical and divide the creature-to-human transformations into two categories. (Immediately I hear the robot from Lost in Space pronouncing “Danger, danger” in the back of my head. Still I will persist.)
The first category is the enchanted beings. These are the humans turned into and entrapped in animal forms. The best known of this category is the beast in Beauty and the Beast. The fox in The Golden Bird jumps to my mind, as well as the bear in East of the Sun and West of the Moon. (You know, no one ever listened to the robot when he declared danger, kind of like Cassandra in the Iliad.)
The second category is the changelings, who can go from one form to another at will, that being part of their nature. These are the mermaids, swan maidens, and selkies.
Already I can think of two variants on the first category. In The Fisherman and his Wife, the flounder is an enchanted prince, who remains enchanted, never resuming his human form. In The Old Woman in the Wood, the prince has been turned into a tree, but can become a bird for a few hours every day, but cannot take on his human form.
(Starting with these two messy exceptions, I think the robot is right. I should listen to him.)
“Somebody tell that robot to shut up!” chuckled the voice from the dark corner.
“But then you have our friend the hedgehog.” It was Megan Hicks entering the conversation.
“He was born half-human/half-hedgehog; and evidently he couldn’t do the full-tilt human shift at will, nor could he discuss it with his bride. He was 100% human for about an hour every night. Also…he was never transformed from human to only sort of human. He was born this way.”
“An odd case, indeed!” Sagacia noted. “Most of those midnight changes are enduring: Lady Ragnell, Frog Prince. But then there are the ones who can’t change back when their false skin is hidden or destroyed.
“I remember Vasalisa telling me about that from the time when she was a frog tsarevna!” said Simplia.
“And there’s the selkie bride whose skin was hidden away and she couldn’t go back to the sea until her skin was found,” Sagacia added. “I wonder if that is what EE in Exeter meant by ‘spell skins’?”
“Then, what are soul skins?” Simplia asked.
From the dark corner, the company noted a slithering noise. A voice said, “Excuse us,” and a frog and a hedgehog scudded across the room and out the door.
There was a long silence. Maybe days. (Jeez, I hope not!)
So, what are they, oh, best beloved? Soul skins?
Fairy Tale Lobby 
Like the selkie stories — A Kelpie who is transformed into a man when his silver neckchain is removed; the bridle in Jane Yolen’s “Witches Ride” which changes any person who dons it into a horse; the woman in “Three Oranges” (Lemons, Pomegranates, etc.) who is changed into a bird when a pin is stuck in her scalp. So many of these stories!
Third category for Charles: actual animals transformed into humans, as in The Cat Who Became a Queen (India), and silly tales of cat brides who can’t forget their cat manners.
If you want to know what a hedgehog soul skin looks like, just do a quick run-through of all the teenage boys you know. Some of the girls too. Very uncomfortable those soul skins are too! But they do seem to rub off eventually, and unlike Hans My Hedgehog, they don’t necessarily provoke those boys into doing ugly things to women.