"The horses are not ours."
Translation:I cavalli non sono i nostri.
It should be. But you might have had a typo or extra space that you missed. Or if you built it from a word bank, there is a bug that will mark you wrong if you drag the tiles instead of clicking/tapping them. Or if it was multiple choice and both options were available, you need to select both, not just one.
I'm not meaning to argue, please don't take it like that, but, if I wanted to say, "The horses are ours" how does not anything else come to play with that? If there was a field of horses, and some belonged to to someone else, wouldn't one say, "Those horses are ours, (or mine), and those belong to Mr. Smith." I guess what I'm trying to figure out, is how many native Italians speak the way these examples are given here on duolingo?
Because articles are just another kind of adjective, and all of the adjectives must agree in gender and number with the noun they modify.
Regardless of whether "we" are all men, all women, or a mix of men and women, it is always "i nostri" because "cavalli" is masculine plural. If we only had one horse (cavallo), it would be "il nostro".
The definite article is required with the possessive adjective, except with singular unmodified family members.
The definite article is optional with the possessive pronoun, although the emphasis changes:
Including or omitting the definite article before the possessive pronoun subtly changes the emphasis:
I cavalli non sono i nostri = The HORSES (and not anything else) are ours.
I cavalli non sono nostri = The horses are OURS (and not anyone else's).