No, although i can understand why you may think so (I'm guessing English is not your native language). In the sentence "This cat thinks that it is a duck" 'it' refers back to the cat. There's no ambiguity here since there's no context to indicate otherwise and no native speaker would interpret it differently. Now if the sentence read "this cat is playing with a toy and thinks it is a duck" then it would be referring back to the toy. "Że to jest kaczka" means "that 'some object' is a duck" so could only be used with the second example.
Der Satz könnte noch ganz umgestellt werden: »Die Katze denkt, dass sie eine Ente sei.« — Das Prädikat muss stets ans Ende des Nebensatzes. :-)
We have to rearrange the sentence completely: »The cat thinks she were a duck.« — The predicate (verb in the sentence) usually has to be placed at the end of a subordinate clause. :-)
cat (male or female, young or adult) - kot
kitten (young male or female) - (mały) kotek
big or small/young female cat/kitten - (mała) kotka
small/dear kitten (male or female) - kociątko/koteczek
female - samica (animal gender)/kobieta (human gender)
male - samiec (animal gender)/mężczyzna (human gender)
feminine noun/grammatical gender - rodzaj żeński rzeczownika
masculine noun/grammatical gender - rodzaj męski rzeczownika
A cat has nine lives (idiomatic phrase) - Kot zawsze spada na cztery łapy
I don't know if the last line can be considered equivalent. Perhaps. I won't argue. But let me just add what those mean more literally:
"A cat has nine lives" = "Kot ma dziewięć żyć" (I'd rather go with plural, "Koty mają dziewięć żyć"
"Kot zawsze spada na cztery łapy" = "A cat always lands on his feet" (four feet, literally).
in Polish sentence "it" is probably the cat, but with some context that could be anything and anyone.
ten kot mysli, że to jest kaczka= This cat thinks that is a duck
ten kot mysli, że jest kaczka= This cat thinks that there is a duck (a duck exists)
ten kot mysli, że jest kaczka= This cat thinks that there (in some place visible on mentioned before) is a duck