Not every fan theory is a serious dissection of lore and continuity; some are simply hilarious. While ThunderCats and ALF are beloved, yet wildly different, properties of the 1980s, it's difficult to deny they were a little silly. So why would one of their fans devise a crazy link between the two that's nothing short of brilliantly ridiculous?
For the (somehow) uninitiated, ThunderCats was an animated series about cat-like humanoid aliens forced to flee their dying homeworld Thundera for Third Earth, where the last survivors continue their fight against their age-old enemies the Mutants of Plun-Darr, and the sorcerer Mumm-Ra. ALF, on the other hand, was a live-action comedy about a sarcastic extraterrestrial (with an appetite for felines) from the planet Melmac who crash-lands into the garage of a suburban family.
The fan theory, put forward on Reddit, brings together the backstories of the two series in arguably the most wonderful way possible. What if ALF (aka Gordon Shumway) and his species weren't simply lovable, furry aliens? What if they were responsible for the tragedy that befell Thundera? In doing so, the Melmacians sparked a war that gave rise to the insidious Mumm-Ra.
In short, the theory goes, the space-faring populace of Melmac, as depicted on the spinoff ALF: The Animated Series, likely would have sent ships to planets other than Earth, such as Thundera. There, a scout would have found (ahem) a ready food source.
Gordon Shumway's species is known to have a taste for cats, so why wouldn't it want to harvest an entire planet of cat people? The conflict would've been vicious, driving the Thunderians to the brink of extinction. The battle would've ended with the Thunderians pushing back the Melmacians, but at a high cost.
In attempting to rebuild, society would be divided into two factions: a conservative group hoping to return to the "Old Ways" of magic and mysticism, and a progressive faction hoping to use Melmac's technology to rebuild society. The ThunderCats would be the product of the progressive society, while Mumm-Ra and his ilk would be the figureheads of the conservative, regressive faction.
Thus, Melmac and their appetite for cats would trigger a planetary conflict.
I Have Become ALF, The Destroyer of Worlds
Before you laugh this theory, consider that Melmac isn't only capable of launching an invasion. It's almost canon.
In the Season 1 finale, ALF tries to persuade the U.S. government to stop using nuclear weapons because his race nearly destroyed itself through its warring tendencies. While it might seem impossible for ALF, in particular, to be the destroyer of worlds, Melmac had enough weaponry to demolish an entire society.
Thundera Is a Dying Society
Furthermore, Thundera is sparsely populated, indicating that much of its population died off in some conflict. It might seem ridiculous to suggest that the Melmac society caused the bulk of the destruction, but what happened to the other people of Thundera?
An invasion by an alien race seeking to feast on the populace would be pretty catastrophic. It's not difficult to imagine a scenario in which the Thunderian nobility is all that remains.
But Let's Step Back a Bit
Here's the big issue with this theory: ALF's probe isn't that high-tech. At least, not high-tech enough to match that of the ThunderCats. ALF's technology isn't that far removed from its 1980s setting (that is, aside from the ship capable of interplanetary travel). By contrast, ThunderCats technology is borderline magic.
This theory is as fascinating as it is ridiculous, but there's not any support for it, even if the idea of the ThunderCats uncovering texts about the Melmacian invasion is pretty funny.