Basically monsters with specific names that are incredibly dangerous and have specific deadly quirks to them making them better against certain foes or just all around dangerous.
some examples
-Skeleton archer-
A highly skilled archer that you would find a hard time shooting back at cause of hard to shoot and if they are not a mele base and they aren’t wearing a riot helmet cause it would gasp lower their accuracy, he might wind up getting a really cheap head shot 
-no idea but seems dangerous-
A enemy that when it sees you would shoot explosion beams that would be 3 wide after you and would only be stopped by walls “destroying them in the process” or when the beam meets its range. and in its spare time would attack bunnys and those other bushy tailed vermin
-Zombie Behemoth-
Zombie hulk times 3 and can and will smash trough most terrain like walls and probably mess up trees and cars carving the most direct path at you.
-shadow stalker-
A giant black spider the size of a horse that has cloaking and really mean poison and would wait for opportune times to nom your face off, like when your on terrain that slows you or when you have low hp or damaged stats.
those are just ideas for creatures that would spawn late game cause a starting character would most likely get pwned by any of them.