If Planet X was visible to the naked eye, do you really think it'd still be unknown to man?
If it's the brightest star and doesn't really twinkle, it's probably Venus. If you've got access to one, get a pair of binoculars/a telescope and get a better view of it. Is it a crescent possibly?
If you're sure it's not Venus.. give us more detail and perhaps we can identify the star/planet/whatever. Whereabouts are you, and where are you looking?