"A cat is an animal."
Translation:बिल्ली जानवर होती है ।
Though एक acts as the indefinite article comparable to 'a'/'an' in English, Hindi uses it very sparingly. It's often omitted and in many sentences, definiteness is inferred only from context. So, the absence of any article in a Hindi sentence may be translated into English as either 'a'/'an' or 'the'.
However, in this sentence, the presence of होती indicates that we are making a general statement. So, 'a cat' and not 'the cat' is appropriate.