Gregar

Gregar, known as Greiga in Japan, is a Cybeast who was born of the same method that made Gospel, which is why they are so similar in appearance. However, the accumulation of bugs was natural and not man-made.

MegaMan Battle Network 6
When he was born, he attacked and destroyed the Net and everything in it. As a solution, scientists invented Falzar to fight it. One day when they were fighting, they got close to the Underground, where they were trapped.

Gregar, along with Falzar was released by CircusMan.EXE and, depending on your version, would go rampant or get caught by CircusMan. MegaMan.EXE tried to stop him by forcibly downloading him. Even though Lan and Yuichiro Hikari try to stop him, he still downloads Gregar, and falls unconscious from the strain. Afterwards, Lan collects Healing Water and uses it on MegaMan to weaken the Cybeast. However, Gregar rejects the water, and takes control of MegaMan. He then attacks HeatMan.EXE and near deletes him. Again, MegaMan falls unconscious, and Yuichiro says that Gregar may take control of MegaMan the next time he fights, and so, may have to be deleted. Later, after working on him, MegaMan gains the ability of Beast Out and soon afterwards, deletes CircusMan with it.

Gregar is the final boss of the Gregar version, and its side of the field cannot be Area-stolen. If you happen to lose Area through use of Colour Point or Double Point, Gregar will not move forward, making Gregar Fire unavoidable. It also appears to be capable of standing over holes. Gregar sports 2500 HP in its normal version, 4000 HP in its SP version, and 5000 HP in RV version.

Attacks Trivia Gregar heavily resembles the Murasame Liger from the Zoids Genesis series as the face resembles it.
 * Beast Pressure: Gregar jumps up and two rocks rain down on your area when it lands. After repeating this for 2-3 times, it will follow up with one of the following.
 * Mane Lightning: Gregar fires its Elec Breath, hitting the front of your area and your current row.
 * Beast Blazer: Gregar fires its Heat Breath, covering the panel immediately in front of it and the back two columns of your area.
 * Tail Cannon: Gregar readies its tail and fires it up and down your column. In the SP version, this attack also tracks your current column as it is in use.
 * Big Claw: Gregar disappears. Then, a 3x3 area just in front of your back column lights up. Its claws will slash that area, while its head will rush down your row. It destroys a loaded chip for every hit. In the SP version this attack will destroy all chips and Program Advances you have loaded if it connects at least once.
 * Mega Beast Tackle: Gregar roars, then charges down its row. Rocks will then constantly fall down randomly on your area, with one always tracking your position.