The difference between " ζώο" and " ζώα" is that the first is singular (animal) and the second is plural (animals).
Αγαπώ αυτό το ζώο. = I love this animal.
Αγαπώ τα ζώα. = I love animals.
Can somebody use the word "ζώο" and refer to a person? Certainly: Humans belong to the animal kingdom, therefore they are "ζώα". However if somebody calls you "ζώο" with an angry tone, it is most probably an insult.