These are OK in various contexts: A cat's biggest enemy is a dog. Cats' biggest enemies are dogs. The cat's biggest enemy is the dog.
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MustAsk Is it possible to say these sentences in a general context? By that I mean cats, dogs and viruses in general:The biggest enemy of the cat is the dogThe human immune system is the biggest enemy of the virusYes, but that is a very formal way. Using the plural is the usual way.