Answer:
I am a kind of coat that can only be put on when wet.
I go up and down at the same time. Up towards to sky and down towards the ground. What am I?
I fly when I'm on and float coming off.
I am so delicate that even mentioning me breaks me. What am I?
Two in a whole and four in a pair. Six in a trio you see. Eight's a quartet but what you must get is the name that fits just one of me.
Dark, feathery, and popular in Baltimore and fantasy books.