User blog comment:Zathronas/Analysis by an author: World of Remnant, Grimm/@comment-10390252-20141017104932

I think that the evidence now supports two related conclusions:

1) The Grimm are, in one form or another, artificial;

2) They were most possibly engineered weapons of some kind.

The question unanswered is, were they specifically designed to attack humans & faunus or were they just designed to attack sapient life-forms with negative emotions and then left behind by their creators as the war was over? If it is the latter, then the technology that spawns them could be eons old and just lay around in maintenance mode until humans appeared and met its target criteria.