Semantics

moustache
Колины гусарские усы были лихо подкручены кверху. Kolya's hussar moustache was curled jauntily upward.
У моего дедушки усы были густые, как у Сталина на портретах, а у папы усы были, как у Гитлера, но перед войной он их сбрил. My grandfather had a thick moustache like Stalin's in portraits, and my father had a moustache like Hitler's, but right before the war he shaved it off.
whiskers
У нашего кота седые усы. Our cat has grey whiskers.
This entry is concerned with the word усы denoting hair on the upper lip of humans or whiskers on an animal. The singular form ус is used to describe the right or left half of a moustache. There are no specific expressions for whiskers on animals. Common sense will dictate which of the expressions can or cannot refer to animals.