|I've got Jack Frost as a cameo role in a children's story |
He's turned up in at least one Terry Pratchett book, if I remember right. As has the Tooth Fairy (in "Hogfather" and others).
I agree that you can't copyright a plot, but I suspect that if you submitted a story to a publisher which was an obvious crib of a well-known (or even slightly-known) book in print, they might point this out in their rejection letter.
Having said that, I believe an experiment has been done whereby a collection of publishers and agents were sent "sample chapters" from a purported new novel, which was actually a word-for-word extract from a well-known novel with the character names changed. Of the 30 or so recipients of this work, only one appeared to recognise it for what it was.