You do need an article like 'a' or 'the' in English. 'Cat is an animal' (with a capital C for a proper noun Cat) would refer to a cat (presumably!) named 'Cat'.
Alternatively you could make a general statement about plural cats: 'Cats are animals'. (Which could also have the definite article 'the', but would then not be a general statement, but rather refer to a specific bunch of cats.)