Onyxia Scale Cloak

The pattern can be learned when you turn in the Head of Onyxia (http://www.wowhead.com/?search=Head+of+Onyxia).

Currently still obtainable in BfA.

Go find [zone=2159] and loot [item=49644]. The head starts A Most Puzzling Circumstance or A Most Puzzling Circumstance which leads you to your faction capital for turn in. After turning in that quest The Journey Has Just Begun (Alliance) or The Journey Has Just Begun (Horde) becomes available. Completing The Journey Has Just Begun (either version) automatically teaches a leather worker this pattern. There is no "pattern" item. So if you're not a LW with the proper level and LW skill level when you complete the quest you don't learn the pattern. I am not sure, but The Journey Has Just Begun probably isn't even available to non-leatherworkers.

I looted [item=49644] and skinned 2 Scale of Onyxia on my first run through the raid with this toon, and had no problem getting either of the quests leading to this pattern.

Yeah, you are. Additionally (to clear things up), if you´ve completed the quest for the head a long time ago, when you hadn´t been LW, you can still go back to Major Mattingly and he´ll teach you.