My thinking behind this is to do with the property of Adamantine Armour which states that "While you're wearing it, any critical hit against you becomes a normal hit" (DMG, p.150).
I wondered if there are any creatures that have this feature "naturally"? - possibly as a result of their skin/scales being so hard or because of another innate feature (but not spell-casting).
I am looking for answers with reference examples of creatures from the 5th edition in the officially published materials.
I'm not looking for a list per se but, if the prevalence of this feature in 5e is not exhaustive, then a list would be fine, too.