It is correct--Duolingo threw us kind of a mean curve ball on this one. Elefant is irregular in that it changes its ending depending on the case. There are very few nouns that work this way and unfortunately you just have to memorize them... For more info check out this thread: http://duolingo.com/#/comment/313?from_skill=0cc096abe097f7dab3bb9434d94ea632
Could you please tell me if the sentence --- It is a dog (Es ist ein Hund) is nominativ or akkusativ ............What I think is if there is a object, then it is a akkusativ, so Es ist ein Hund is correct instead of Es ist einen Hund. It is nominativ according to me.
Now why do u say, He has a elephant is akkusativ?