Green Winter Hat

The Green Winter Hat was a novelty hat, but was changed to Cloth armor as of Patch 2.3.

Source
During the Feast of Winter Veil, the Green Winter Hat is a very rare world drop for humanoid mobs.

At the same time, the hat is also a Boss drop for specific mobs based on which expansions the player owns.

For those that own only the basic game, the hat drops from:
 * Darkmaster Gandling in Scholomance
 * Goraluk Anvilcrack in Upper Blackrock Spire
 * Prince Tortheldrin in Dire Maul West

For those that own The Burning Crusade, the boss drops change to:
 * Grand Warlock Nethekurse in Shattered Halls
 * Grandmaster Vorpil in Shadow Labyrinth
 * Exarch Maladaar in Auchenai Crypts

For those that also own Wrath of the Lich King, the boss drop changes to:
 * Mage-Lord Urom, the third boss of the Oculus

For those that also own Cataclysm, the boss drop changes to:
 * Drahga Shadowburner, the third boss of Grim Batol

In 2007 the hat dropped from all six Azeroth and Outland bosses regardless of expansions owned.

In 2010 the hat dropped from Mage-Lord Urom even for players who owned Cataclysm.