The bible talks about "great beasts' outside the garden of eden. Plus time is different, before the flood for example humans lived 800+ years. And for God a day could be eons, or it could be a millisecond.
The devil is not the opposite of God. He is the opposite of Michael. Satan is a fallen ANGEL. Saying that he is the opposite of god is false and puts Lucifer on the same level as God, which is wrong.