Den and det in Danish
you know how 'it is an egg' translates to 'det er en æg', would the same for a common noun, for example, 'it is a/the cat' be 'den er en kat/katten'?
"It is an egg" translates to "Det er et æg".
Regarding your actual question it is a bit tricky and depends on context. But in most cases you would use "Det er en kat"
Example 1: "What is that behind the bush? It is a cat" Translation: "Hvad er det bag busken? Det er en kat"
But if it has been clearly established in an earlier sentence that we are talking specifically about a cat, then you can also use "den" (although it is optional).
Example 2: "Why does a cat meow? Because it is a cat" Translation: "Hvorfor mjaver en kat? Fordi den er en kat"
Conclusion: If in doubt use "Det er..."