"Cats are not food."
Translation:고양이들은 음식이 아닙니다.
이 isn't a marker or anything added to 고양, the whole word for "cat" in Korean is simply "고양이". It's a coincidence that it ends in "-이". It's like how the word for "vacation", "휴가", just happens to end in "-가". In this case, -가 doesn't actually mean anything; it's just part of the word. If you were to make "휴가" the subject of a sentence, it would indeed get another "-가" on the end, THIS time as the subject marker. (Reminder that it's -가 and not -이 because it ends in a vowel sound.) It would end up as "휴가가". The first -가 is just part of the word; the second is the actual subject marker particle, stuck on the end for grammatical reasons. Hope this helped!