Blackjack - A Ranch Dog by Thomas C. Hinkle

Blackjack's mother was a huge black dog, probably part Northfoundland, and so Jack was also a huge dog.

Clark Preston claimed Jack as his own and the pair did a lot of hunting together. Wild dogs were a menace to the cattlemen in the area and Blackjack proved that he could stand up to any of them on the day of reckoning.