What requirements do I need to fullfill to transmogrify into a legendary look?

When you Identify a legendary post 2.0, the appearance is unlocked in the Mystic's Transmogrification. Any items identified PRIOR to the 2.0 patch will NOT have their looks unlocked. To unlock them you'll have to find a new copy of the item. Further, if you craft a legendary/set item (after the 2.0 patch) its appearance is also unlocked.

From a blue post on the Diablo 3 forums:

Now, on to business. Legendary items found in patch 2.0.1 will unlock as Transmogrify options for you in the expansion. That includes the ones you identify, so feel free to do so! Once you upgrade your account to Reaper of Souls and have access to the Mystic, you should see the Legendary items you found in patch 2.0.1 as available Transmogrify options. Normally unlocking a Legendary Transmogrification option will be notified to you, but the ones found (and identified) between patch 2.0.1 and Reaper of Souls will not.

I should note that legacy Legendary items (those found prior to patch 2.0.1) do not unlock Transmogrify options

Once you identify a Legendary item, that look is unlocked to transmogrify.

In your case, I would assume that at some point you had those shoulders.