The Hang Mobile (ハングモービル,Hangu Mōbiru?) is a boss from Sonic & Knuckles, as well as its locked-on form, Sonic the Hedgehog 3 & Knuckles. It is fought in the second act of Flying Battery Zone, after the ship falls apart.
The Hang Mobile uses slightly different sprites for Robotnik than other bosses in the game. The sprite is smaller than usual, and faces head-on to the screen rather than side-on. Furthermore, only the Robotnik sprites were adapted in this way for the boss battle - no Eggrobo sprites were adapted to fit the Hang Mobile model. Therefore, even in Knuckles' story, the pilot of the Hang Mobile is Robotnik, not Eggrobo. This is the only boss in the game in which Knuckles fights Dr. Robotnik.
Sonic and/or Tails
Dr. Robotnik will move the Hang Mobile in the direction of the player. Occasionally, the mobile itself will swing round a total of four times. The mobile can be hit once with each spin, with eight being required to destroy it. Jumping between the spiked clamps will cause Robotnik to fire the flamethrower on top of the mobile, but can be avoided by jumping away quickly.
As usual, this boss is harder for Knuckles. Dr. Robotnik will periodically fire the flamethrower even when Knuckles is not standing above it. In addition, the mobile only spins round twice each time rather than four times, so the boss takes longer to defeat. As with Sonic and Tails, eight hits are needed to destroy it.