R. M. Ballantyne 
The Dog Crusoe and his Master [EPUB ebook] 

After winning a shooting competition, Crusoe, a Newfoundland dog, finds himself the best of all conceivable friends. Huge, like other members of his kind, and trained for two years to become an excellent hunter and swimmer. Crusoe was prepared for anything; his tail and ears would instantly rise.
Crusoe, Dick, and two other brave explorers are followed as they trek the western plains on a hazy peace mission among the local tribes. As well as some of the most hostile ‘Red-skins, ‘ there are grizzly bears, a stampede of wild horses, avalanches, and whirlwinds.
A little boy won Crusoe and a weapon in a shooting competition. The reader gets to experience the thrill of adventure as the kid and dog mature and rode out to see the wide prairie for the first time as well as the many creatures and wonderful landscape.