Wildlife can be very dangerous to your dog. Deer can kick a dog to death. 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.