I love pony mysteries!!
I wouldn't be surprised if it was from a fakie line or another Hasbro line - they do love to reuse accessories.. I wonder if it could be some kind of plastic breakdown, though? How does her plastic feel? I know I've heard of the colour leaking out of Baby Katie's Kitty and turning her orange instead of pink, but I can't remember if that makes her plastic hard or soft.. if that makes sense
I could be totally off here but anything's possible w/ ponies, haha.