Crimson Longinus
Legend
Giant snapping turtle has natural armour AC of 17 so exactly on point lower than full plate like I suggested. Not that I think you should really try to derive things from monster stats that way; they're not designed to be material components for crafting.Turtles don't have high enough AC.
How they are unnatural? Perhaps they're just naturally occurring denizens of a fantasy world, just like elves, tabaxi and what have you.Umber Hulks and Bulettes are unnatural, which makes them at a minimum as bad as metal armor.