Dentaegrub

Armor: As Unarmored (Juvenile), As Leather (Adolescent), As Chain (Adult), As Plate (Ancient)
Hit Dice: 1 (Juvenile), 2 (Adolescent), 4 (Adult), 10 (Ancient)
Hit Points: 3 (Juvenile), 7 (Adolescent), 18 (Adult), 65 (Ancient)
Move: 3/4 Standard (Juvenile/Adolescent), 1/4 Standard (Adult), Minimal (Ancient)
Damage:
Gnaw: 1d6-1d10 for Juvenile through Adult, 2d12 for Ancient
Drop (Adolescent Only): 1d8-3d8, depending on falling distance.  If the victim takes more than 1/4 of their HP at once, Save vs Paralysis or be dazed for 1d4 rounds.
Tentacle Snare (Adult and Ancient): Save vs Paralysis or be immobilized while the creature approaches.  Attacks against a snared target have advantage.
Snare and Drag (Ancient): As Tentacle Snare, except the Ancient bug is strong enough to drag its victim in.  If two victims are in close enough proximity, it can grab both at once.
Number Encountered: 1d4 adults, 1d6 Adolescents, 1d10 Juveniles, 1 Ancient.
Experience Points: 25 each (Juvenile), 50 each (Adolescent), 150 each (Adult), 1200 (Ancient)
Morale: 5 (Juvenile), 7 (Adolescent/Adult), 10 (Ancient)

The sound hurts your ears.  Mankind evolved to hate the sound of scraping enamel, and the creature's casing drags across the concrete floor, grinding over jutting pieces of rusted metal and rubble.  It is still growing, and needs to add to its home-armor hybrid.  You have up to 32 of its preferred construction material right there in your mouth.

The dentaegrub itself is a soft, fat, translucent maggot, ranging in size from a softball to a mastiff.  It eats carrion, and anything slow or stupid enough to wander into reach of its sharp mandibles.  Adolescent grubs start building themselves a protective casing using their favorite light, strong, hard material: teeth.  Secreting a sticky cement from a long, whip-like tentacle that emerges dead center from the forehead, they pack the teeth together into a shell that they expand as they grow.

They’re not picky.  The teeth in their armor range from tiny rodent teeth, all the way to the massive curved saber-like teeth of the flying beasts that terrorize the surface.  And of course, they’re fond of the pearly-whites in your own skull.

Small grubs can hide easily in rubble and ambush passing rats.  Basketball-sized specimens tend to attach themselves to ceilings and drop onto prey.  Larger bugs drag their bulk behind them, shrugging off attacks as they devour carrion, or corner slower, less tenacious creatures, snaring them with their now quite sturdy tentacle if they can manage it.

Some tunnel-scouts tell rumors of dentaegrubs so large, that they cannot move, and lie heaving in lairs, dying slow, terrible deaths.  One unhinged man spins wild tales of a cult that worships such a foul thing, and brings it sacrifices, as though it were their fat, immobile god.