"The dog is not scary."
Yeah. It's getting annoying that the rules that the app sets up to follow drastically are suddenly dropped
There doesn't seem to be a big difference in meaning. Maybe くありません is a bit more formal. I too thought that くないです is more common, at least in spoken Japanese. Google it, lots of discussions out there... :)
As far as I understand, 怖くないです is less formal, while 怖くありません is more polite. But they mean the same thing.
It's just a negative form of こわ.い Drop the い and add くありません. Thus, it doesn't have anything to do with the dog.
Maybe this is more like "there is no fear" rather than "there is no scary dog"?
I wrote いぬはこわくないです and the correct answer it gave me was いぬはこわくない. Was I too polite, or not polite enough then?