Other mistake: When Cloud Dancing and Daniel walk a wounded Sully to a clearing, we see a road with what looks like flattened black tents on either side, and Cloud Dancing tells them the Army must have attacked it; that's what is left of a reservation. When it's a modern transport road for everyday access, very poorly disguised as a reservation camp.
Continuity mistake: The gown Gilda S. Claire wears at her open concert is not the same one Emma made for her in the Mercantile.
Answer: A Cooper was someone who made various things out of wood including wooden caskets and even barrels.