first, sort out \GameMechanics\Creature\Creatures\Necropolis\Walking_Dead.xdb
the folder name Necropolis needs to be changed to Neutrals, and the file Walking_Dead.xdb should be renamed to Creature_777.xdb — insert whatever number you need; there's a list of taken numbers on HC, though it doesn't include numbers for creatures from our forum((
if you're not planning to release your mod publicly, you can take any number (numbers start somewhere around 160 and go up to 999)
now you need to open the stats file and remove the dependency on original creatures — your zombie upgrades into a plague zombie and is registered in Necropolis; the fields <PairCreature> <CreatureTown> <Upgrades> need to be rewritten for neutrals; any elemental will work as an example of an independent creature
there's also a small flaw
the paths to MapObject and CreatureVisual are written with an error
<MonsterShared href="/MapObjects/Dracul/Ghost.(AdvMapMonsterShared).xdb#xpointer(/AdvMapMonsterShared)"/>
<Visual href="/GameMechanics/CreatureVisual/Creatures/Dracul/Ghost.xdb#xpointer(/CreatureVisual)"/>
all paths lead to the creature Ghost, but your creatures are called Zombie_Warrior
now go into \MapObjects\Dracul\Zombie_Warrior.(AdvMapMonsterShared).xdb and at the very bottom rewrite the line <Creature>CREATURE_WALKING_DEAD</Creature> — replace WALKING_DEAD with the number
now you need to sort out \MapObjects\_(AdvMapObjectLink)\Monsters\Necropolis\Walking_Dead.xdb
rename the Necropolis folder to NCF and the file Walking_Dead.xdb to Creature_XXX.xdb (by the way, inside there's the same error with Ghost and Zombie_Warrior)
you also need to deal with the camera (it's missing in your mod) and the script file (that's also missing) — download psatkha's zombie mod and pull the scripts and Cameras folders from there; you'll need to replace the numbers, and in the script file write down the name