Great ideas on the forum:
To expand on a few:
(a)Since you have something to melt leather armor - how about the other side of the coin - Rust Monster?
(1) Melee attack has %chance to destroy unit metal weapons and armor
(2) Magical weaponry and armor has lower % chance to be destroyed
(b)Vampire (/or Dryad?)-
(1)Mesmorize/(Kiss) - Melee attack on unit % chance to change alliegance of unit for remainder of fight (if unit lives - reverts at end of combat)
(2)Fear/(Hypnotized) - Melee attack has % chance to root/paralyze unit
(3)Will only exist/travel in wooded squares (vampire would need protection from sunlight, Dryad would need tree)
(1) % generated every time someone develops a metal mine (new mine, new tunnels)
(2) Every turn (or group of turns) left unmolested has % chance of destroying resource (raze?)
(3) Immune to non magic weapons
(4) Does not leave mine
Anyway, I'll see what you think of these ideas (and if the restrictions on some of these work) and post more.