Seven Elder Demon Emperors (七魔皇老, Shichima Kōrō) are the seven demons created from Anos's own blood 2,000 years ago, just before he reincarnated. Their existence was created and gave life by magic, not by mating. It was prepared because Anos needed a body to inherited his own blood when he reincarnated.
The Seven Elder Demon Emperors are the 7 demons created from Anos's blood. They inherited the traits and talents of the Founder. Before Anos reincarnated he commanded these demons to have children and expand their bloodlines. These 7 demons started the Demon King Academy Delzogade to raise the next generation of Demon Emperors. Their followers and subordinates started calling them Seven Elder Demon Emperors since Anos didn't give them names because he was too busy with his reincarnation.