Here's how to unlock some creepy zombies and dead stuff:
Sewer Zombie: Complete Fight Off The Living Dead(Challenge/Behead the Undead) and get Silver or higher.
Crypt Zombie: Complete Casualty (Amateur League/Beginner's Series) and get Gold.
Gargoyle: Complete Cold Corpse Caper (Honorary League/Maximus) and get Silver or higher.
Undead Priest: Complete Stain Removal (Challenge/Glass Smash).