For your consideration, here is a collection of monsters taken from various movies, TV tropes, literature, and miscellaneous spooky stories, re-tooled for insertion into your 5E game. Where everyday is Halloween!
Apocalypse Zombie (CR 2) - Fast, ravenous zombies that spread like an epidemic.
Blightwood Snag (CR 18) - A destroyed treant revived by necrotic energies.
Dracossomagi (CR 25) - An undead ancient dragon skull.