The way I make sense of this question is that there is someone who doesn't know what a Viking is. Then they ask if the Viking is a kind of cat. Like ragdoll is a kind of cat.
If the T and V were capitalised this could also be a question about a weird looking pet called "The Viking".
I'm not an expert on grammatical terms and someone might give you a better answer, but in this sentence viikinki is the subject while kissa is the predicative expression, based on the word order. If you swap the two, "Onko kissa viikinki?" kissa becomes the subject and viikinki the predicative expression, and then the sentence would mean, "Is the cat a viking?" If you want to say, "Is it a viking cat?" you need to add the pronoun "se" in the sentence and use a compound word, viikinkikissa, "Onko se viikinkikissa?"
I've tried to make sense of this frase and my interpretation is that maybe somebody has heard of vikings, but doesn't know what they are. And then that someone wonders if a viking is a cat. And he would get an answer - no, vikings are not a cats but ancient scandinavian sailors and merchants. I don't know...
The most people can expect out of duolingo are incredibly stupid sentences like this one that DON'T help you learn in any way. Their playfulness is best kept for the playground. Such a sentence is completely useless when one tries to learn a new language with the limited resources that this app provides. Awaiting the salty downvotes, even though this is the truth.