Shadow Raiders/WMG

The Beast Planet is Earth in an alternate Mortal Engines where the plan came to fruition.
After London destroyed all the other cities, either with the Wave Motion Gun or by eating them, it eventually covered the entire world. Then it dug into the Earth's core to harness the heat. With powerful engines, it devoured the entire Solar System, ending with the sun itself; then it went on to become the terror of the galaxy...

Bone's political instability is because it is the most independent of the planets.
They have organic technology (which lessens a need for minerals), huge stockpiles of explosives (which may indicate a source of energy), food, and likely water to grow said food. Because they don't necessarily need to import or steal nearly as much, their ruling class doesn't need to have the unified front that the other societies need as to not get wiped out.

The Beast Planet is Unicron.
First up, the manner in which the Beast Planet eats planets Tek, Fire, Jungle, and Reptizard is highly reminiscent of the way Unicron at Lithone in Transformers the Movie. Also, the planet Botanica explored to get her plant mode looked very like Planet Jungle; while this may simply have been a simple case of recycling an environment to save on costs, it's much more fun to think the two series take place in the same universe. And if Shadow Raiders and Beast Machines exist in the same universe, that means Unicron must be out there somewhere.

Also, Unicron seems to have some new power every time it appears, so Beast Planet's ability to create drones isn't a strike against this theory.

A fun theroy is that the Beast Planet is what became of Unicron after his failed attempt at resurrection in the G1/Beast Wars timeline. His body was revived as a soulless machine who continues to consume planets but no longer has Unicron's consciousness driving it.


 * Considering Unicron hops between universes at will, it might not be all that unlikely that Unicron simply decided to stop by the Shadow Raiders universe for a day and eat their reality.

The Cluster have met interstellar populations before.
Cryos in the first episode mentions that the Beast Planet is a myth, a boogyman to frighten small children. How the hell would they have heard about it if they didn't have some contact with someone who had some contact who had etc. etc.
 * I'd actually take it even further. The 4 planets are unable to remain independent without each other, so I think they were probably created at a initial level of space faring capabilities. Then those precursors left and that part of the history of the 4 planets disappeared. So long as the precursors knew about the Beast Planet, they could tell the inhabitants of the system about it. If you wanted to take it further, you could make the argument that the Beast and the Precursors are inherently linked - either the same entities or separate parts of the same faction. Could be similar to the Burning Legion from Warcraft who destroy the creations of the Titans while the Titans pay them no heed, or it could be that the Precursors create planets for whatever reason knowing that the Beast will eventually consume them - perhaps intending them to eventually be consumed by the Beast - perhaps even being the Beast and this merely being what the Beast does when it is done with a system. Either way, the Beast is intentionally targeting creations of the Precursors - how else would Lamprey know that all of those planets have a World Engine - not just a possibility, but a reality. Heck, she knew it well enough that Tekla was able to find the entrance to it in under a day.

The Beast and the Ancient Race are the same
At some point or another, the Ancients who created the world engines and teleporters became the Beast Planet. At some point in the distant past they transformed their world into a single massive weapon of war, which, given their world engines, would be child's play since the Alliance already did that to their own planets in season two. The Beast/Ancients just took it to the next level. They built it up bigger, armored it, armed it, and now they're on a rampage. Or, alternative, they created the Beast as a kind of weapon--perhaps they were fighting, and even losing, an interstellar war and were forced to create such a weapon to defend themselves...and it didn't work. Like at all. In that case the Ancients may have actually engineered the warlike Cluster and their neighbors on other planets specifically to fight the Beast, like a failsafe.
 * Maybe they're the same species, but rival factions.

The Beast Planet is a Reaper.
Black with red lights? Basically invincible? Completely unstoppable, boasting weapons more powerful than any other species'? Stick a few tentacles on it and we've got a Reaper.
 * The Beast Planet is an alternate universe version of a Reaper, where instead of harvesting living beings, it harvests entire planets. Good thing there's only one.
 * The Beast's Generals are either smaller Reapers, or indoctrinated from another race that it consumed eons ago.