Wild horse

This story is about a young man (Ben is about 13) and his younger sister, Dixie (love her independent spirit!) who monitor a wild horse herd several miles away from their ranch, in the wilds of Idaho. A new stallion has taken over, a huge black horse with obvious good breeding. They name him "King", and are cheered by the idea of his progeny helping to improve the "broomtails/scrubs" breeding of the area. But when men arrive with intent to either capture or kill King, Ben and Dixie try to come up with a plan to save him.