Rogue Survivor/YMMV


 * Demonic Spiders: Zombie Masters are this for having high HP, decent speed, being able to open doors and jump over obstacles, and leading groups of undead to swarm you.
 * Bikers also fall into this category, as they attack in coordinated teams, can open doors, are more than capable of getting past barriers that would stop zombies, and actively hunt down survivors (like, say, the player character) and supply caches (like, for instance, the food you have squirreled away).
 * Fortunately, bikers, unlike zombies, have to eat and sleep, and they can't track you by smell.
 * High Octane Nightmare Fuel: The Hospital and Sleep music. "Happy Nightmares" indeed.