Mustangs are highly sought after for working cattle. The healthy, young and strong wild horses aren't used for food, they're trained and sold to work ranches.

I don't know of any widespread trade of salt in pre Columbian Midwest. However, salt is readily available across most of it from my understanding. There are several salt mines around the great lakes currently, the biggest I believe being under lake Erie. If I'm not mistaken there was also a lot of salt brought with the glaciers, which is why so many rivers in Illinois have consistent salt licks along their banks. I don't know nearly as much about the plains regions west of the Mississippi, but would guess that it's similar.

There are a lot of "salt Creek"s in the region. Most are nam d Salt Creek because of the natural salt licks along their banks. A quick Google search I found several papers discussing salt usage and harvesting in the Ohio River valley.

I know from having lived in Illinois a lot that the Rock River, Fox River, Vermilion River and several others have natural salt licks. What is now Sterling, IL was a Sauk village before European settlers. Part of why they settled there was access to salt and the game it attracted. Lincoln, IL along a Salt Creek is the same, although it was a Kickapoo village. Danville IL as well along the Vermilion River.