Supposedly White Croats lived about that area over 9000 years ago. I think Nestor chronicle (maybe it's called Primary chronicle or something like this) notes them. Not sure if it shows in toponyms for example, or in archaeological remains (as far as I know Slavs about those times belongs in the same archaeological culture).
Not sure their relation to "normal" Croats. I also dunno if their language was preserved in any form.