I understand why this is "Cats are beautiful animals" ... But I wrote, "cats are a beautiful animal" without thinking why this wouldn't be in a language course. What I wrote is a much more abstract phrase, but one that would still be used in English with the same meaning.
Only if there was a species specified, though - "They are a beautiful animal" would indicate "they" is in a singular form. But since "cats" or "wolves" cannot be a singular form the rest of the sentence, whether plural or singular, is understood as plural.