Get up, come on get down with the Phoenix
open up your items and point that finger to me
Down as in the feathers? Well I'd say Down Feathers for plural, I wouldn't pluralizes Down. So I just say Phoenix Down. Maybe sometimes Downs but not really.
Cannot answer without context. For instance:
1 Phoenix Down or a stack of 99 Phoenix Down, as well, because down = feathers, already plural.
If you're specifically talking about feathers from more than one Phoenix, then Phoenix Downs.
If you're talking about Daniel's backup cosplay for the 2016 EoFF meetup:
Phoenix Towns.
If you're talking about Russel Crow kicking ass in the arena:
Phoenix Frowns.
If you're talking about the cartoon:
Felix Meows.
If we're talking about seeing your parents scrogging:
Penis Down.
If you're talking about Botticelli:
Venus unsuccessfully covers her Mounds.
If you're Bad Luck Brian in the fountain of youth:
Fetus Drowns