Giant Crab
Born from the depths of the sea, these creatures have enlarged over centuries at the depth of the sea. Notably, they're renowned for their enormous claws which once used to be for merely crushing simple bone that has now evolved into being able to crumble stone and whales alike. They are not necessarily known to be ferocious, yet they are defensive and protective of their land and kin.
The skin, or 'armor', of a giant crab is almost equal in strength to that of iron due to its enormous husk -- though this is mostly due to the weight and size it is bolstered with coupled with the muscle beneath and to try to form it into human-wearable equipment would prove to be an extremely arduous and difficult task that would be prone to chipping and, with exposure to air, weakening over time.