The first, and so far still only, piece of legendary armor dates back to Vanilla and more specifically Molten Core. It was a crazy time back then, with two tiers dropping in the very first raiding instance WoW has ever had, an instance that already had two legendaries. So, why didn't you know about this little gem of an item, still one of the most notable pieces of loot ever found in the game?
Well, probably because it only ever dropped once, for one person, in the entire history of the game. That's right, the only person ever to have this little gem was Noktyn from the guild Nurfed. And I thought Ashkandi had a low drop rate back in BWL...
So yes, clearly this was a mistake and the item was never supposed to drop in the first place - Vanilla was a chaotic and messy time for itemization, drop rates etc, nothing was set in stone yet and the developers were playing around with a bunch of different concepts. The item itself may not seem like much now, but back then it was perfect for tanking, with the constant (if tiny) damage output and the solid primary stats, not to mention the only two resistances that were relevant in Vanilla. And the icing on the cake was that it also came with its own special animation added to your character:
The neck was immediately hotfixed off of Baron Geddon's loot table after it dropped, never to be seen by another soul - except for Noktyn, who was allowed to keep his, making him probably the most unique character in WoW history. Unfortunately the character isn't viewable in the armory due to some server transfers and inactivity, so those two screenshots are all we get! - Update: Thanks to Thornass we got his armory link!
So why did this even exist? It probably had something to do with Bindings of the Windseeker, as it dropped from Geddon and it's name had Binding in it! But thematically it would make sense, now the big question is whether it was actually what Thunderfury was supposed to be (which would make sense since there already was Sulfuras as the legendary weapon in the instance) or if it was part of a more complex quest chain to get Thunderfury. In any case it's one of the more interesting and certainly unique stories from back in the day and still, to this day, the only legendary piece of armor to grace the World of Warcraft.