Trash Contract Beast? My White Fox Has Demon Blood

Finn Carter goes through the contract ceremony and comes out of it bound to an F-rank white fox, the sort kept for looks. Everyone watching laughs, and by the rules of the place a summon that weak puts him in line to be sold into slavery. The Divine Evolution System wakes in him while that is being decided. It reads the fox and finds the bloodline of the nine-tailed celestial fox, one of the ten ancient demon beasts, under the white fur, and the sky and the ground react to the reading. On the strength of that Finn is given a probation period instead of a sale, and sent to work the black stone mine. He completes the tasks the system sets him. The fox moves up to E rank, and part of the taming energy comes back to Finn, who is stronger after it than he was before.