I'm guessing Fallen Chi-Yous. There was this similar thread before asking for a *Raven* Gun, so I suspect it should drop from the same Aragami.
The Japanese Wiki says to break its Arms for the feathers. It's fairly common at 50% drop rate.
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DJ. INFRTaku BS- Beginner CE-Grand Master DMP2-Guest
A sole petal descends into the darkness, but no matter how deep it falls, it will never be alone...