Cześć Heesob :-) First of all, we would rather use only one question for both singular and plural: Co to jest? Co to jest? To jest kot. Co to jest? To są koty. We don't ask: „Co to są?". Secondly, „to" in the question „Co to jest?" is not a demonstrative pronoun. It's just the personal pronoun of the 3rd person singular (English: „it"). The other „to" (same as „ten" and „ta") is a demonstrative pronoun (English: „this"). Take a look at this 2 sentences. Can you see the difference in meaning of these two „to"? To jest kot. To dziecko jest małe. I hope I could help.