I believe Dr. Robotnik was a human, but when he died at the end of Sonic the Hedgehog 3 & Knuckles, a robot named Dr. Eggman took control of his brain to keep his evil schemes going. Basically, Dr. Robotnik was a human, but not Dr. Eggman. (Robotnik was palamigan anyways.)
He's human the way Sonic is a hedgehog. If you go outside and look at a hedgehog, it won't be blue and run really fast. What I'm saying is, Eggman may not be an actual human like Chris, but he's the equivalent of one in Sonic's world.