I don't know if theres any bosses in the game, its more of just like 400 zombies coming at you I think. "The Tank" zombie is somewhat of a miniboss, he does some massive damage and takes a few clips to kill. Luckily in the demo you only fight him practically solo. But him busting down a door and letting dozens of fast zombies in, shit would be bananas.
Well I'm not sure if you know what I mean by boss zombies or if you've heard how the game's AI characters/zombie human characters are described so let me explain.
Other than the typical zombies the game has 4 boss zombies, Hunters, Smokers, Boomers, and Tanks. In co-op and mp play for the most part you can only play as the first three, then occasionally someone gets to spawn as a Tank.
Hunters are stealth bosses that are very quiet and can go invisible when not moving while near a wall. They have a pounce attack that is tricky to pull off as they don't jump at but onto their prey.
Smokers have a 50' prehensile tongue they can shoot out and grab their prey with. They can also give off smoke to hide in, thus the name.
Boomers are big fat guys full of methane that vomit on you, which temporarily blinds their victims. If a Boomer is shot they explode violently and can take out the whole crew of survivors.
Obviously you already know what the Tank is. These are the 4 bosses that I meant would be AI when playing sp. At least that's the way I thought the sp would play, kinda disappointing if not.
There are also witches that are AI even in co-op and mp. They are fairly quiet and still until someone gets near them, then they start freaking out and attack if you have a light on or make too much noise.
Valve says the witches are only present at higher difficulty levels, but as far as I know the boss zombies are always there, in co-op and mp anyway. It could be in sp the boss zombies are only there in higher difficulty levels too though. They don't ever really talk about sp play much in the dev interviews and videos, so it's hard to get info about.