The Rust Monster for OD&D

The Rust Monster 

Oldest version of this critter I could find is from Supplement I Greyhawk 1976, it appears also in Dungeon Master's Kit #1-Palace of the Vampire Queen 1976. 

Rust Monster

No. Enc: 1-2

Alignment: Neutrality

Movement: 12

Armor Class: 2

Hit Dice: 5

Attacks: 1 touch

Damage: special

Save: F5

% in Lair: Nil

Treasure Type: Nil

These seemingly inoffensive creatures are the bane of metal with a ferrous content, for as their name implies they have the effect of rust upon such substances, and this happens nearly instantaneously. Any hits by or upon a Rust Monster cause even magical weapons to rust and fall to flakes. Armor is affected in a like manner. The creature is very fast, being attracted to the smell of the iron-based metals, and when alone it will devour the rust it has caused.

Dungeons & Dragons OD&D rules, Supplement I Greyhawk, Illustration from the Monster Manual