Wesen (the creatures of "Grimm")

I’ve watched every episode of Grimm, read all the way through the Grimm thread, read the Wikipedia entries on the show and the wesen, and read through too many pages of the Grimm Wiki. I still seem to be missing something.

Are all of these creatures (“wesen”) the product of the writers’ imaginations – based loosely on Grimm fairy tales, of course – or does this concept come from some pre-existing books or stories?

As a “furry” I can’t help wondering if they didn’t come across a website I used to visit which had porn for just about any were-creature you could think of.

I think the idea is that they’re supposed to be like the talking animals in the tales, but they’re people too.

At first, I thought they were playing off of “were” creatures, but the wesen breed true, don’t convert others by biting them, and can control their conversions. I can’t find any legend or lore about these critters, so I guess they are the product of the writers’ imaginations.