4th Age, year 1824. Date unknown.
The more I see of the behemoths, the more impressed I am with the diversity of the sub-strains within the same race. The analogy I gave of dogs is more appropriate than I had imagined; numerous creatures of various sizes and psychological characteristics are all part of the same genome and share only a basic shape. The latest strain brought to my attention is one I have affectionately dubbed the 'bulwark beast' on account of its hard, rock-like outer shell, and the adorable shield-like mandibles it employs to protect itself from incoming attacks. These creatures are essentially invulnerable, so long as their shield remains intact, with only magic having even a modicum of effect on them before the shield is removed. These creatures make excellent guardians, so, I shall place them in areas of the dungeon that we no longer concern ourselves with inhabiting, in order to ensure that even the areas we have long forgotten pose a challenge to interlopers.