Skyrabbits (monster for BX & 5e)

Midnight-blue rabbits whose lustrous fur twinkles with motes of light. They can burrow through any earthly material, their bodies subject to no mortal poison. They live in the firmament, grazing on moonsilver vegetation growing inside the cavernous pinprick holes that let in starlight from beyond.

"When I was a wee boy, the sky fell.

"The sages say an armada from beyond the stars came to breach our "crystal sphere", whatever that is. Whatever they were, they failed, but their cannons or whatever must have blasted fragments of the sky itself down here. Now the sky grew back, else I wouldn't be here, but those fragments were riddled with skyrabbit warrens.

"They burrow through anything with ease: wood, stone, even steel. Our world must be nothing to paws and teeth capable of digging through the sky itself. Rumour has it even the wizard Arthras, one of the few still able to conjure the fabled Wall of Force, was unable to repel an infestation from his demesne. Supposedly his tower stands abandoned, its foundations doomed to collapse any month now.

"Their fur's easy to spot, and they're worth a small fortune even once the twinkle wears off. But not even wolves can catch them, and they shrug off any poison. Caught one in a trap once, but it just gnawed through the metal, leaving only shining purple blood behind. Occasionally some big military commander captures a few to dig into an enemy castle or something. But the breeding stock always gets loose again, letting the bastards loose on yet more soil.

"There's a ranger stalking these parts, follows the Old Ways. She says the world itself rejects these beings of aether, that they cause reality to erode. I've seen it, I think. The soil near their warrens, just... decaying into its fundamental parts. She says that if they're not stopped, eventually the skyrabbits might destroy the world as we know it.

"I'll be long-dead by then though. Just m'bones, drifting through the stars. Sounds neat."

Skyrabbits in B/X

Note: [square bracketed] stats are for use with systems with ascending AC and/or unified saving throws

Armour Class: 7 [12]
Hit Dice: 1⁄2 (2hp)
Move: 150’ (50’), 60’ (20’) burrowing
Attacks: 1 × bite or kick, 19 [+0] to hit
Damage: 1
Saves: as normal human [18], Death 14 Wands 15 Paralysis 16 Breath 17 Spells 18
Morale: 5
Alignment: Neutral
XP: 5
Number Appearing: 1d3 (3d6)
Treasure Type: None

• Flighty: Unless magically summoned or controlled, skyrabbits check morale every round.
• Glowing pelt: Sheds light for 10 ft. Continues glowing for 1d6 days after death. 
• Immunity: Unaffected by common poisons.
• Keen smell: Automatically smell silver, otherwise only surprised on a 1.
• Peerless burrower: Can burrow through any known material, including magic forcefields.

Skyrabbits in 5e

Tiny beast, unaligned

Armour Class 11
Hit Points 3 (1d4 + 1)
Speed 40 ft., burrow 20 ft.

STR     DEX     CON     INT     WIS     CHA
4 (−3)  12 (+1) 12 (+1) 2 (−4)  14 (+2) 14 (+2)
 
Damage Immunities poison
Condition Immunities poisoned
Senses darkvision 30 ft., passive Perception 12
Languages --
Challenge 0 (10 XP) 

Illumination. The skyrabbit sheds bright light in a 10-foot radius and dim light for an additional 10 feet. The pelt continues to shed light for 1d6 days after the skyrabbit dies.

Keen Smell. The skyrabbit has advantage on Wisdom (Perception) checks that rely on smell, and automatically smell silver.

Peerless Burrower. The skyrabbit can burrow through any material, including walls of force and similar effects created by mortal spellcasters.

Actions

Bite or Kick. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 1 piercing or bludgeoning damage, as appropriate.