Bears can maul a dog to death. Raccoons will tear a dog up and can have rabies. Rattlesnakes can inject poisonous venom. Bobcats can kill a dog. Mountain Lions prey on dogs.
Rodents can carry a number of diseases.
All of the above carry parasites ranging from fleas and ticks to worms of various types. These are good reasons not to let your dog off leash. Northern Breeds will eat large herbivorous mammal's feces. Feces can have numerous types of parasites. If you are in an area with horses, cattle or deer, your dog will need to be watched closely or it will eat the feces.