Board Thread:General Discussion/@comment-3257939-20140105070225/@comment-3146930-20140718210055

Another idea if that they are called the Creatures of Grimm because they are the physical manifestation of grim (the double 'm' being a nod to the Brother's Grimm). If you look up the definition of grim, it turns up words like:
 * forbidding
 * formidable
 * ill-tempered
 * ferocious
 * threatening
 * implacable
 * black/dark
 * ect.

It actually describes them pretty well.