Pull down the notification bar and see. If this isn't your phone, we can't help you. Doing so would be an invasion of privacy and against the forum rules.
As Javier mentioned, it's most likely the new Gmail app notification
The first time I saw the new Gmail app notification, it kind of freaked me out too, but pulling down the notification shade I was able to quickly determine that it was in fact the new notification for the Gmail app.
Just as an aside...I personally don't like the new Gmail app notification, but ohh well
Welcome to Android Central! I moved this from the Android Apps forum to the General Help forum since the former is intended for developers to promote their apps.