Planet X (Cybertron)

There is no longer any such place as Planet X. It continues to exist solely as a codeword, an identifier for an underground army of loosely-affiliated Transformer criminals. The true number of "Planet X's" forces is unknown, and many of their identities are similarly shrouded in mystery.

What is known about Planet X is that the warriors, thieves, assassins, and other dubious characters who claim a link to it are the survivors of a dead planet. The Manifest Destiny of Planet X was to subjugate countless worlds, and it did, enslaving the natives and feeding the whole of their conquered worlds piece-by-piece into the World Furnaces that powered it. But their expansion ended at contact with Gigantion, the Giant Planet. As a result of the battle between the two planets, Planet X went nova. The few survivors, driven mad in their quest for vengeance, made a deal with no less a force than Unicron himself, offering their services in exchange for the destruction of the hated Giant Planet.

The Decepticons Sideways, Soundwave and Soundblaster are currently the only known Planet X operatives. However, swarms of bird-drones left over from the planet's mass armies still exist, and are just as dangerous as they were back when Planet X was whole.

History
In an alternate universe, Planet X was the most well developed world in their universe. However, it soon ran low on energy, so the inhabitants had to colonize somewhere else. They happen to come across Gigantion, which was pulled into their universe. But things proved difficult in colonizing, as the Transformers of Gigantion used their Cyber Planet Key to double their size. Soon, it was apparent that Planet X would lose the war, forcing them to construct more dangerous weapons. When they built a devastating planet gun, it backfired and destroy Planet X.

(Note: It is theoretically possible that earlier Unicron Trilogy characters with Unicronian affiliations, such as Nemesis Prime could be connected to Planet X.)

Galaxy Force
According to the booklet included with the final Japanese Galaxy Force DVD release, Planet X was actually the body of Unicron himself from a parallel universe. Megatron's absorption of Unicron's power caused this body to go into a frenzied repair mode, attempting to sustain itself. In the process, it resurrected several of the Transformers Unicron had consumed to manage its frantic systems - these beings became the inhabitants of Planet X, and for decades, they attacked other worlds and absorbed them for fuel on a mission of galactic conquest. Eventually, Soundwave discovered Unicron's massive memory bank buried deep within the planet, and began studying it. This became their downfall, however, when they crossed paths with the Giant Planet, and used what they had learned to unlock the full power of their planet - destroying it and themselves in the process.

An official timeline on the TakaraTomy website would later clarify details of the conflict with Gigantion, explaining that Planet X actually pulled it into their own universe, with the intention of stripping it of resources and making it their new home. The timeline dates the destruction of Planet X to the 6th Century - so precisely how Megatron's absorption of Unicron's power in 2030, causing the creation of Planet X, could have happened before this, is... perhaps best not thought about too hard.