It is possible to drop personal pronouns in Greek because verb conjugations are indicative of the person and the number used. For example, there is no need to say "Εγώ είμαι, εσύ είσαι" ("I am, you are"), simply "Είμαι, είσαι" suffices. Therefore, including personal pronouns is completely optional. When included, they give emphasis to the subject: Εγώ είμαι εδώ, πού είσαι εσύ; (I am here, where are you?). Of course, the verb ending for the third person is the same for he/he/it and in plural for they masc./fem./neut. and that can create a bit of confusion sometimes, but in the sentence above the subject is obvious.
And a final note: αυτός, αυτή, αυτό are both personal and demonstrative pronouns, meaning "he, she, it"or "this" respectively. It is therefore possible to translate both "it" and "this" with "αυτό".
Greek is a null-subject language, because its verb conjugation allows to understand the subject even if it is absent. Είναι is "he is" "she is" "it is" or "they are". From the fact that you talk about an animal you get to understand that it means "it is" in this sentence.