Hang Mobile is the main boss of Flying Battery Zone. It is battled in Sonic & Knuckles and Sonic 3 & Knuckles.
Curiously, the boss displays as Robotnik for Knuckles, and not the Eggrobo who is the main nemesis in his storyline. This is likely due to the fact that the battle's perspective is different--the Eggpod and thus its pilot face the screen head on, and so require new sprites, which the artists did not implement for EggRobo. Knuckles was also not originally intended to visit Flying Battery Zone before the level was moved to Sonic & Knuckles. The hack Sonic 3 Complete by Tiddles addressed this by implementing sprites edited from one of the ending scenes.
The pre-boss consists of the player being trapped in a laser beam-walled area, much like the boss from Wing Fortress Zone, only the moving laser on the roof fires a blast, then the room gets smaller and smaller, with Robotnik/Eggrobo working the controls to the right of the room. The blasts are fairly easy to dodge - just stick to one side of the room, wait until the device stops moving and then spindash out of the way - and due to the short-sightedness of the fat madman, the dodged blasts have been tearing his ship apart. The laser walls eventually drop and Robotnik/Eggrobo runs off to the right, and as the player gives chase the wall slams shut behind you and the ship starts to fall. After a short chase sequence, the main ship completely disappears from sight, and Hang Mobile drops in from the sky, swings around a couple of times, then begins to attack.
This boss has two main attacks - he swings around in front and behind of the main platform, causing damage if the player is standing in the centre of his attached limbs; and when he is hanging below the platform, he shoots fire from the top of his head and shimmies left and right, his spiked limbs causing damage if they touch the player. When playing with Knuckles, the machine shoots fire all the time, instead of shooting only when the player (be it Sonic and/or Tails) stands over the machine. The only opportunity to hit him is when he is swinging around - when the boss is underneath the platform, he is invulnerable. After the usual 8 hits, the machine falls to pieces - but beware the spiked limbs, as they stay deadly for a few moments longer than the rest of the boss does. Robotnik flies off to the right, past the Capsule for the act. Once broken, the character runs to the right and spins though an access hatch, sending it flying and the character falling - straight towards the desert of Sandopolis Zone.