"Un animal plus ordinaire, il n'y a pas."
this sentence sounds weird in English. Couldn't you write it as "There isn't a more ordinary animal"? or something along those lines?
If you were a translator and not a Duolingo learner, yes your suggestion would be better.
It seems to me that this is a literal translation, so the English translation sounds very french. I agree with jakesail that "There is not a more ordinary (or common) animal" would be better, even as a Duolingo learner. Learning a language is not only about learning words and literal translations, but also about learning/ understanding what the differences between languages are. In my opinion the skills of the Duolingo learner/ translator should also be taken into account: the current translation helps you to learn the words and understand what is written, very suitable for a beginner. In more advanced levels I would suggest a translation that is more related to the spoken and/or written language.
ordinary = common So, what's wrong with this translation? A more common animal, there is none. (?)
I don't understand why this translation is wrong: A more ordinary animal, there isn't one.
It is wrong because Duolingo is not smart enough to realise that its translation is bad English. It only accepts literal translations; not the real world ones. Have the same problem all the time!