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.