What native american group developed the american appaloosa breed of horse?

The Nez Perce. They bred a superior breed of horse, the Appaloosa. It wasn't long before the Nez Perce gained a reputation in the Northwest for their hunting skills, horsemanship and craftsmanship. Appaloosas have a reputation for strength, durability, and large spots of black, brown or other colors against white patches.

To be honest i thinks its between the first answer and the third answer

From mexican american war , us gained new mexico.