Unkown contacts

I see many UNKNOWN contacts in my Android (Galaxy s4) MAIN PHONE CONTACT LIST (not the Whatsapp contact list). These unknown contacts only have WhatsApp green icon next to them, and no phone number, etc. I do not know these people. First of all, I am concerned about how did all these contacts appear in my phone contacts list and how do I get rid of them besides deleting them from my main phone contact list? They are ONLY listed in the main contact list of the phone (not whatsapp contacts list, they do not show in whatsapp contacts list if I search for them