마리 is the counter for animals.. 개 한 마리 ..one dog.. 곰 새 마리 ..three bears..

Where is the "only" in the korean sentence?

I tried 'We have just one dog' and it was incorrect. Should that be accepted or is it too far from the translation?

Apparently, I can't say "a dog" Duolingo wants me to say, "one dog." It's the same thing. At least, means the same thing. A dog, one dog. I missed this the first time just because of that.