Is "Co te są?" incorrect?
As I know, "Co to są?" means "What are these?"
And the plural of "to" is "te".
Why not use "Co te są?" instead of "Co to są?" ?

Question

Is "Co te są?" incorrect?
As I know, "Co to są?" means "What are these?"
And the plural of "to" is "te".
Why not use "Co te są?"  instead of "Co to są?" ?

Answer

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.