There are Bosses that may be encountered. Each of them must be defeated in the same playthrough.

Zombie Bear
Encountered during scavenging. Killing it will require a lot of ammunition. Keep a wall between yourself and the bear and wait until the scavenge ends. Keep your distance and kill any other zombies present before focusing on the bear.

Zombie Dogs
Encountered during scavenging. You will be attacked by two zombie dogs along with regular zombies. Each dog requires three hits to take down. They will lunge to attack and move quickly. Get some distance before attacking them. When they crouch, it indicates that they are about to lunge and that you should run away. If possible, you can also try to wait until the scavenge ends.

Zombie Deer
Encountered while travelling. You will hear a rumbling sound and see walls of deer will approaching your car. Keep your car in good condition and stay near the middle of the road when possible to avoid them.