Translation:This is mine.
That should be correct if I remember my Japanese lessons correctly.
Given that the particle の indicates posession, saying "この犬は私のです" should mean "This dog is mine." The もの in Duolingo's example sentence seems redundant if theyre saying the English translation is correct. That means it should be "これは私のです。"
Thanks. I had the same question. Korera would clearly be plural, but Duo frequently allows plural references without the "ra" or "tachi." I didn't realize that Kore was unique. [And I do notice below that others suggest that "my thing(s)" should be acceptable. So apparently I am not the only one confused.