Bone Feather

The odds are thin enough that going after this specifically is a bad plan. Armored Bones only turn up in the Dungeon once Plantera is down, and that same farm is already handing you the spirit drops and the rest of the post-Plantera loot, so treat the feather as a bonus from a setup you were building anyway. A water candle and a battle potion in a cleared bone brick corridor is the usual arrangement, and a later patch improved the rate, so it stings less than it used to.

It exists to make Bone Wings and has no other use. If you already have something better strapped to your back, craft the wings anyway before selling, since the finished pair fetches considerably more than the pile of parts that went into it.