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 in the officially published materials that have this feature "naturally"? – possibly as a result of their skin/scales being so hard or because of another innate feature.
I am looking for answers with reference to creatures from 5e or previous editions.
