BUT IT MAKES SO MUCH SENSE!

We know that Grimm are attracted to things with souls, and would aim to eat humans. If we then extrapolate and assume Grimm gain their animal motifs because that's what they've consumed before (lesser souls in the form of animals/wildlife) then the Faunus could have popped up shortly before the advent of Dust, in the form of super-strong Grimm who have eaten enough humans to gain a soul of their own and a human-ish form!