In American English penguin costume and penguin suit are two different things. A penguin costume is a costume in which you are dressed like a penguin. A penguin suit is slang for a tuxedo.
For once, American English and British English are the same. If you are wearing a penguin costume, you are dressed as a penguin. If you are wearing a penguin suit, you are dressed like the Penguin.
Could I respectfully correct an incorrect notion in this forum. As a Brit 'of a certain age', I can vouch for the fact that, in the UK, the term 'penguin suit' does NOT imply a tuxedo ( or, as we put it, a 'dinner jacket'). A 'penguin suit' implies full morning or evening dress, including, crucially, a long jacket WITH TAILS. That is the similarity that spawned the name. Such dress is now frequently worn in the UK for weddings, and for very formal occasions, such as royal investitures, etc.
So is a "costume de pingouin" a penguin suit (dinner jacket) or a penguin costume (penguin fancy dress)? I have assumed the latter but the thread here suggests not (?)
He is wearing a penguin suit.... Why is this not penguin suit. Costume implies fancy dress.
Context. If it is intended as a masquerade, it is called a "costume". It doesn't imply that it is fancy at all.