The Green Hill Zone boss is mostly a regular Egg Mobile but it moves around using helicopter rotor blades in addition to the usual Egg Mobile's jet propulsion. It also has a giant white boomerang attached to it. It is also more armored than a normal Egg Mobile, with a protective roof and glass cover on top of the engine. In terms of coloration, it is mostly grey with the Egg Mobile's usual yellow and black hazard stripes.
Powers and abilities
Robotnik's vehicle is capable of flight and can deploy deadly boomerang-based projectiles.
In Sonic Blast, Dr. Robotnik used this vehicle to attack Sonic and Knuckles when the duo was passing through Green Hill Zone. In spite of Robotnik's efforts though, his vehicle got destroyed by the two heroes in the end.
When the player arrive in a wide empty area at the end of Green Hill Zone Act 3, Dr. Robotnik arrives from the top-right corner of the screen. After a few seconds, he will start making his vehicle's boomerang spin at an increasing speed before launching it in a diagonal low curve pattern toward the lower-left part of the screen before the boomerang goes off-screen. The boomerang will then come back to Robotnik from the upper part of the arena. He will then use again this attack before moving his vehicle towards the left corner of the screen and go off-screen, before coming back, this time toward the top-left corner of the screen and starting the whole process over again (albeit from the opposite direction each time) until he is defeated.
This boss needs six hits to be defeated. However, those hits must be dealt precisely on the protective glass of the Egg Mobile. Trying to hit this boss vehicle from any other spot will simply bounce the player back without dealing any damage (and potentially put the player at risk of being caught in the boomerang pattern).
After being defeated, the Green Hill Boss will blow up before disappearing in a flash of white, clearing the Act.