I don’t believe children are natural liars but they naturally believe that whatever is expedient for them is “right.” If the truth served their purposes, they would tell the truth, and vice versa. It is only by learning to get out of oneself and think abstractly about principles and fairness and long-term consequences (which definitely needs to be taught) that children learn wisdom.
A natural liar is simply what you describe. Someone who lies without having to be taught to lie doesn’t mean that they always lie. Even Democrats don’t always lie (sometimes they aren’t talking).