AskHistorians

In Latin, "Graeculus" and "Brittunculus" - technically only diminutives, but derogatory in-context. Often translated as "Greekling" (I prefer "Greekoid") and "Nasty little Briton"

Greeks and Romans used "Butter eaters" and like terms for people who ate foods that were 'foreign' to them. Or simply who ate more of a particular food - the Romans in particular regarded excessive meat-eating as barbaric.

References to national dress were sometimes used, like trousers or headwear.

You could also speak of national (supposed) attributes, like "Punic faith" for someone who is untrustworthy - ie they can't be trusted to keep faith when pledging their word, like the Punic peoples.

Latin is very uh, 'rich' with terms and idioms that can be used to abuse someone on the basis of gender or sexuality. We'd be here all night. Recognizable to modern eyes, Julius Caesar was accused of taking the 'passive' role in a homosexual affair with the King of Bithynia, and so earned the mocking title 'Queen of Bithynia'.