"Cats are animals."
Translation:Οι γάτες είναι ζώα.
Because you need to be specific. Greek and English have different rules for this case. While in English the article is ommited, here the article is necessary. You're talking specifically about cats, generally being animals. It might sound a bit odd, but that's the logic behind it. That's why the article can be ommited in ζώα but not in γάτες. Also, you can't ommit the article in such a sentence, at the beginning of it. ^.^
Well, technically, τα γατιά could be kittens, both female and male in gender. Οι γάτες are female adult cats, while οι γάτοι are male adult cats. ^.^
(Although, keep in mind that these two (γάτες-γατιά, even γάτα-γάτος-γατί) are often interchangeable. One may have a male cat but still refer to it as "η γάτα μου".)