I was wondering where Roberta might have heard of these obscure stories in the pre-Internet age, but then I realized that the Mannikan/Mannikin story and the Rosanella story are both found in Andrew Lang's Green Fairy Book (first published in 1892). They're even both by the same author. (And wow, the story about the white snake [the same thing that Crispin gives Graham in KQ5] is in there too!)
Interesting observations I had actually noticed the Green book connection between Heart of Ice and Rosanella. Didn't know about the white snake though!
I've been thinking more about Crispin though. The KQC suggests that Crispin may or may not be a persona he has adopted and that he may or may not have once been Merlin and that Cedric is Archemedes.
Well in KQ6 we learn of yet another mysterious wizard which some think died, others think poofed himself into an Aardvark. He was the only wizard he land knew! There is also a myth that the Isle o Wonders was created by a wizard according to the Guidebook, but I'm digressing.
Could it be possible that Haroun alRashid was just another persona for Crispin? He neither died nor shapeshifted. This could explain why he knew of he Islands location!
In KQC the court wizard (Haroun?) knew were Serenia was, and sent Derek there from the Green Isles!
If the wizard is Crispin/Haroun, it would certainly explain why he would both know where Green Isles is, and also why he would know where Serenia is!
but the KQC's statement that fairies (in the KQ universe) must spend one day a week in the form of an animal is a concept I haven't come across in any traditional fairy tales yet
As for the one week thing, I'll look into it, there must be some kind of inspiration behind it. Most of Peter Spear's material has something inspiring it. If I discover anything I'll post my findings. It would be funny though, if it turned out to be another idea taken from the Green book, or the other fairy books by that author!
Edit: Well, I did come across this reference to a 14th c. french fairy tale, something about the Legend of the Fairy Melusine.
She was a fairy who was married to a human. She would transform into a half-serpent creature one day out of every week, in this case because of a curse.
It certainly seems to share similarties to the rules mentioned about fairies in KQC, and could be the inspiration for the idea (there may be similar stories elsewhere though). See here
Edit 2: Apparently, the idea of shape shifting Snow Leopards is a common theme in ancient myths... So Peter could have gotten that idea from assortment of stories. See here
, for more information!
Seriously don't underestimate Peter Spear, or the various KQ authors for dredging up obscure Fairy Tales and sources of myth!