Rust Strider (5e Creature)

Large aberration, neutral

Armor Class 20 (natural armor)
Hit Points 37 (5d10 + 10)
Speed 30 ft., climb 20 ft.

Saving Throws Str +5, Dex +2, Con +4
Skills Perception +1
Proficiency Bonus +2
Damage Vulnerabilities lightning
Damage Resistances cold, fire; bludgeoning, piercing and slashing from nonmagical attacks that aren't adamantine
Damage Immunities poison, psychic
Condition Immunities charmed, frightened, poisoned, unconscious
Senses darkvision 60 ft., tremorsense 60 ft., passive Perception 11
Languages —
Challenge 1/2 (100 XP)

Rust Metal. Any nonmagical weapon made of metal that hits the rust strider corrodes. After dealing damage, the weapon takes a permanent and cumulative -1 penalty to damage rolls. If its penalty drops to -5, the weapon is destroyed. Nonmagical ammunition made of metal that hits the rust strider is destroyed after dealing damage.

The rust strider is a mysterious insectoid creature of metal held together by seamlessly integrated organic matter. Its dominant feature is six long legs each ending in massive curved plates infused with various metals. These plates act as the feet with a small plume of hair like setae at the end to attach to most surfaces. The legs attach to a narrow but sturdy thorax that supports a large diamond shaped abdomen of similar metal composites as the legs. The head is similar to most any insect but with only two black eyes. A small beak serves as a mouth, splitting open to reveal several feathery brushes it uses to break down and slurp up food.
The diet of the rust strider mostly consists mostly of metal with the occasional need to eat soft decaying flesh. The average person might confuse this insectoid creature to be a rust monster, but the rust strider has a much smaller appetite. What metals it does eat are broken down and used to strengthen the exoskeleton which becomes difficult for any predator to crack. The few who do manage to see the insides of this creature will find only lean muscular fibers and a network of tubes serving as its nervous system and veins. There are no complex organs let alone a brain.
The rust strider is a mindless creature. Its behavior is that of a drone. It only responds to clicks and vibrations created in its tunnels by its immobile queen. They seek out food and use their hard bodies to deter predators from their nests. If need be the rust strider can pull itself into a ball to protect itself further, taking any eggs it can inside itself. Very few creatures find them worth eating, but smarter creatures have found a way to control the rust strider through tools that simulate the clicks of the queen.
Many races of the under dark have captured and trained these creatures to serve as mounts using a clicking gauntlet that can be felt by the rust strider's tremorsense. Even in heated combat or magical silence, a rider can guide their mount. Normally a rust strider egg sells for 200 gp. A full grown rust strider sells for 600 gp. Though most of the cost is to the command gauntlet. The rust strider can not be trained.