I don't mean to be critical, but wouldn't this sentence make more sense if it were "Η γάτα είναι ένα κατοικίδιο"?
Yes, it would make more sense, but Duolingo courses don't have only logical sentences. Sometimes there are sentences that doesn't make sense or puns or references to songs and famous quotes in order to gain learners' attention :)
it doesnt make sense in turkish aswell. I didnt understand why they have some nonsense sentences
On the bright side, they help us remember because of how weird the sentences are. XD
It actually makes perfect sense, since cats tend to own their human slaves, not the other way around, despite what they allow us to think ;)