I don't know what kind of costume you need... is it for a boy or a girl?
Some suggestions would be
Alligator
Anteater
Apple
Angel
elephant?
any Apple product, really, haha. iTunes, iPod, iBook...etc.
queen
cat, catwoman, captain america, captain crunch, carnage, cameraman, captain planet...
SUPERMAN, Scheherazade
Noddy / Navy / Ninepin
Skeletor costume starts with s.
that's a hard one... try a music fairy or something or an Egyptian mummy
astronout, archeoligist, artist, (think of any jobs starting w/ a and u will most likely be able to come up with a costume) Hope I helped
Answer Kings and knights come immediately to mind. For the macabre, a killer might also work.
Panthers Pumas Pirates
Jackie Chan jet lee jack skeloton Jason (Friday the 13th) Jesus christ jack and Jill Japanese person