WhatsApp mobile instant Messenger which was initially release on the January 2009, it was founded by Jan Koum and Brian Acton. In February 2014 Facebook acquired WhatsApp and now managed by Facebook. WhatsApp Messenger is a freeware, cross-platform and end-to-end encrypted messaging application for smartphones and is user friendly.
WhatsApp now has new features in it which you can use to have your privacy by hiding your online status, last seen and profile picture. This how you go about it.
Follow The Steps.
Tap on the WhatsApp Messenger when it open
Go to Settings => Account => Privacy
And then tap on Last Seen => Nobody
Now that you have hide online status on WhatsApp, If you later want to change back, go through the same steps and change to Everyone.