With the announcement of Elder Scrolls Online going into beta testing, I've taken a look back on the games so far and realized that each of the last 3 games have included special armor types either in the core game or their expansion packs that are not seen in any of the other games.

Throughout Morrowind, Oblivion, and Skyrim, these are the special armor differences I have witnessed.

Morrowind = Chitin Armor, Bonemold Armor, Pauldrons, Full Face Helmets, Ice Armor (Bloodmoon Expansion pack),

Oblivion = Mythril Armor, Crusader Armor (Knights of the Nine expansion pack), Madness Armor (Shivering Isles Expansion Pack), Amber Armor (Shivering Isles expansion pack)

Skyrim = Dragon Armor, Ancient Nord Armor (after finishing Companion quest chain), Wolf Armor (after joining Companions, bought at Skyforge)