From Sonic Retro

The Green Hill Zone Boss, in which Dr. Eggman attacks Sonic with a large ball attached to a chain. It is perhaps the most famous boss fight of the series.

In the early games, at the end of each zone Sonic would face either a robot or Dr. Eggman controlling a machine. As time has worn on and new games and scenarios have been added to the series, the boss fights have become more and more varied! Often, Robotnik remains the villain, piloting one mech or another, but now Sonic and friends have faced many other kinds of foe in boss battles.