There are several stories, I think.  People's souls are animals, and in one of the stories Dean's and Sam's souls are switched (wee!chester, I think).  I thought I'd bookmarked them, obivously not.   Somebody's Dark Somethings are the books, I think. Thanks!

Edited to add:  It's HIS DARK MATERIALS and I've found a bunch in the Tags-- anymore new recs would be welcome!