"This cat thinks that it is a duck!"
Translation:Ten kot myśli, że jest kaczką!
"Ten kot myśli, że to jest kaczka" would work (I just added it). It just means that the cat thinks that [it/this/that] (something that he looks at?) is a duck.
"to jest kaczką" doesn't work.
The Polish sentence means that the cat thinks of itself as a duck. That it has identity issues.