"I a tortoise" is a little flowery for the average English writing - more like how a poet or academic might write. Not that' it's wrong in the context of the translation.
Either comma splicing isn't a thing in French or someone at Duolingo is grammatically challenged
Turtle vs. Tortoise: http://wiki.answers.com/Q/What%27s_the_difference_between_a_turtle_and_tortoise As a non-native speaker of English I found this very interesting. As far as I can tell it's the same as Landschildkröten and Meeresschildkröten in German.
It's actually a bit more complicated than that. The usage of "tortoise" and "turtle" depends on the dialect of English being spoken. See the wikipedia entry here: http://en.wikipedia.org/wiki/Turtle#Turtle.2C_tortoise.2C_or_terrapin and these Oxford English Dictionary definition: http://oxforddictionaries.com/definition/english/tortoise?q=tortoise and http://oxforddictionaries.com/definition/english/turtle?q=turtle for more on the controversy.
Duo corrected me saying "He's a duck, I've a turtle" was correct. To me "He's" means "he is" which would be wrong and "I've" just sounds like slang. Very strange.
I've means I have(Though it looks sort of odd to me, even though I'm a native speaker) and he's can be he is or he has.
In the audio version of this, the un and une totally get lost unless you play it in slow motion, which I don't really want to be doing, because a native french speaker wouldn't speak that slowly.
Couldn't 'it has a duck' work too? Just wondering, since that is usually the one I use when I see 'il'.
In English, this would be a grammatically incorrect run-on sentence. Are such run-ons allowed in French?