Is it possible that the pelts aren’t even tanned? Rather, stretched and dried?
If they were intended to be sold to a tannery, that would have been the way they were stored prior to sale, in which case, you’ll have to have them tanned.
You might try a couple things yourself. Take some Neats Foot Oil, or Saddle Soap and apply it liberally to the skin side of the hide. Roll them up tight and leave them in a warm place 90-100 degrees for a few days to let the oil soak in. Then you have to break the grain of the hide which is done by constant stretching and working.
I’ve broken deer hides by putting down a deep layer of straw, hide, burlap, more straw on top and repeatedly dropped a big rock on it. Takes forever!
All that said, if they were mine, I’d be off to the taxidermist and have them done commercially.