Shelmet (5e Creature)
From D&D Wiki
Small monstrosity, any alignment
Overcoat. The shelmet can breathe normally in any environment, and it has advantage on saving throws made against harmful gases and vapors (such as cloudkill and stinking cloud effects, inhaled poisons, and the breath weapons of some dragons).
Shell Armor. The shelmet takes half the damage from critical hits. This does not prevent any additional effects due to critical hits, such as that of a vorpal sword.
X-Scissor. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) slashing damage.
The typical shelmet is slow, dumb, and its attacks aren't very powerful. However, its thick shell armor makes it difficult to attack even the most exposed of shelmets. Even so, most of them lose their shells eventually and instead of seeking a new one, they evolve their other points and become an accelgor.