Fighting Predators
There are several ways to get away from predators and avoid a confrontation. Running away only typically works when the predator is still a ways off. A decoy almost always works unless the animal has already started charging at you. Waving fire at them usually works if you're patient and not advancing on them.
Wolves
When wolves attack, they can be fought off. If you have a hunting knife, hatchet, or heavy hammer in your inventory, it will automatically be equipped when you're attacked. Having one of these tools makes it easier to fight them off and more critically wounds them. Of course, the best option would be to kill them with a weapon before they get to you, but that's not always possible or practical.
Note: A wolf scared off by fire or the sound of a rifle report may attack you if you're still nearby when it calms down.
Bears
Bears cannot be fought off once they attack, and should never be engaged close-quarters. If you find yourself an unlucky victim of a charging bear, the only thing you can do is lay there, take the mauling, and try not to soil yourself. Bears take several shots to bring down, even in the head.
For more on hunting predators and game animals, see this page.