As a general statement that both species are animals, English requires either the plural (Cats and dogs are animals) or the definite article with both nouns (The cat and the dog are animals). "Cat and Dog are animals " would seem to refer to a specific pair of animals named or called "Cat" and "Dog". "The cat and dog are animals " would most likely refer to a specific pair of animals. It could conceivably be a generic statement with the article intended to apply to both nouns, but that would only be clear from context. In any case, Duolingo's consistency with its use of the English definite article doesn't determine what is or isn't idiomatic in English. I suspect, but do not know, that the usual English for what the sentence means is "Cats and dogs are animals. "
"Cats and dogs are animals" is the best correct answer, but at the same time we should give the Duolingo volunteers who produce these courses some leeway. They are native speakers of a language that doesn't pluralize in the same way that English does, and we native speakers of English should be aware of how challenging our system can be for them. Let's offer our considerate opinions that it could be brushed up and be grateful that we're helping each other to keep on improving our second+ language(s). (^-^)v
Duo's consistency with its use of English articles does not equate to correctness in the use of English articles. There are aspects of Duo's use of English articles that are consistently wrong, e. g., the use of "the" with abstract, uncountable nouns.
What irritates native English speakers is not that Duo gets English articles wrong. It is that Duo often rejects the correct usage.
It doesn't matter whether Duo Lingo is consistent or not. English is English and is not determined by Vietnamese grammar. Strictly speaking, Vietnamese has no noun declension and no definite or indefinite articles. Arguably, it has no plural in the sense that declined languages do. In translation what matters is the meaning rather than the form or structure.