You’re not getting answers for 2 reasons:
- the question is still somewhat vague despite the valiant effort to describe it!
- randomisation like this is easy to find and has been covered too many times.
For example:
or, if the monsters are of the same class, and all that’s needed is swapping out their static meshes:
And there sure is a dozen or so other methods, some better than others but all depend on the details, scope, existing script. But perhaps this gets you started.
3rd reason
Or no one cares 
Next time consider attaching a screenshot showing how the monster is being spawned atm. This way we can suggest something that fits right into the setup you already have. Otherwise we must guess how you could be doing things and can merely suggest something disappointingly generic, as above.