Board Thread:Episode Discussion/@comment-24993958-20161231141844/@comment-28288412-20170101002618

So the Grimm were created prior to humanity for the purposes of destroying all forms of life. But, some time after humanity came into existence when Grimm set their sights on man, they only attacked humans and faunus and their creations exclusively.

I would ask why, but I do have an assumption. Salem, whatever the reason and method, she is responsible for this focus.