Prototype Bub, (also known as Mechanical Prototype Bubblun, Mechanical Prototype Bubblen and Bubblun Prototype Code No. CR20 - 100190), is a large, evil robotic bubble dragon that made its first appearance in Puzzle Bobble 2. It is the body guard of Dreg, and is operated by Dreg from the inside chest plaste which acts as a cockpit. It is referred to as a large battleship, which is a reasonable title considering its nature. It protects Castle Dreg at the end of Player VS Computer Story Mode in Puzzle Bobble 2. It has a single yellow spike on its head, almost trying to pass as a real bubble dragon. It has no legs, having jet boosters in place of them and large claws on the end of silver extendable arms. It also has maroon pauldrons on its shoulders and a single red eye from which it sees through. Upon its appearance in Puzzle Bobble 2, it scares Bub quite badly. He is 1 of 2 characters that can't be played as via normal means.
Prototype Bub has its own unique bubble machine. Instead of an arrow, it uses a metal cannon. It seems to be very adept at using this, though it is generally harder for others to aim with it. Prototype Bub also has a unique condition that allows it to be on the top layer. This means that bubbles that descend down the screen become covered up by its huge frame, making it impossible to see what colours they are. This does not affect it in the story as it is computer controlled, but by the use of Character Building Cheats, playing as Prototype Bub becomes considerably difficult.
The Prototype Bub returns in Pop'n Pop as Dreg's method of transportation and brings him to the match before the battle takes place.
While it does not serve any other purpose, at least it confirms that it is still in service and functional after the events of Puzzle Bobble 2. It's worth noting that Prototype Bub is not being used as a weapon in Pop'n Pop as Dreg clearly exits it before the battle begins, unlike in Puzzle Bobble 2 in which it is used in a threatening manner (most probably because in that game, it is defending Dreg's Castle from Bub.)