Sonic Unleashed Movie?

[Vipershark]

Japanese Robotnik was always Eggman. The US manual (and subsequent Comics/Cartoons) called him Robotnik because there was no such thing as canon back then (or at least they could retcon shit without complaint).

Then eventually (by Sonic Adventure 1), the US writers were called out on the difference, and the names were homogenised. Eggman is now Robotnik's 'nickname', and Mobius is Earth.

In SA2, it's mentioned several times that his real name is Dr. Ivo Robotnik (when his grandfather is onscreen, he is also refrenced as a Robotnik.) IIRC, in SA1 they said Eggman was just a nickname.