Well, there's this one. http://archiveofourown.org/works/5598295 I doubt it's what you're looking for, but one's a horse, and one's a unicorn. :)

Thanks i will go and read this now.

There's Gone with the Wendigo (http://archiveofourown.org/works/426340), where they're turned into My Little Ponies. It's a decent case-fic, as well as hilarious.

Thank you.

Too Big for This World http://krisomniac.livejournal.com/113612.html
Summary, A/N: Sam has occasion to recall his brief yet blissful time as a mythical, white stallion (Horse Dean)

Of Unconventional Dealbreaking http://rionaleonhart.livejournal.com/145255.html
Summary: Dean, much to his annoyance, gets turned into a unicorn (Unicorn Dean)

Thank you i will go and read these now.