Horned Rat is a magical construct of the Old Ones.

[The Cunning Fox (raZoleg)]

…and a deity they have manufactured to create the biological weapon — Skaven. Skaven transmute warmbloods into a minimum-maintenance, highly numerous army, that maintains quality by constant infighting for supremacy between each other (strongest survive — a biological selection to make sure only the best warriors make it), yet stays relatively monolithic, through the guidance of the council, without the grudges between elves, dwarfs and man.

Skaven are omnipresent, a feature important for the guerrilla warfare Slann and Old Ones were fighting in, as their war became essentially a war of attrition, and they became like the occupied country, as chaos beasts rolled in. Skaven hate the clans that sided with Nurgle, two civil wars have proven it, yet they diversify their own population by making, essentially, warrior castes that each excel in one thing. But, much like the Lizardmen — they have lost their aim, like their god, and therefore exist as a part of a machine that keeps going by itself, an engine that works, but without a propeller, for example, much like Lizardmen — without a purpose (except their prime directive — absorb everyone who is not them and destroy as long as they exist).

Skaven’s affinity for magic, their ability to make biological weapons (Moulder) and their hunt for warpstone (destroy your enemy’s fuel source, and you destroy him) make them a perfect anti-Chaos army.

[RedDevil99]

Skaven didn't appear until many thousands of years after the Old Ones had vanished, can directly communicate with Daemons of the Great Horned Rat and even the GHR himself and most importantly Pestilens is not aligned with Nurgle and the civil wars they fought with the other Clans were for political power struggles only (at least on the other Skaven's side).

[Zav]

Originally posted by razoleg (The Cunning Fox):

…and a deity they have manufactured to create the biological weapon — Skaven. Skaven transmute warmbloods into a minimum-maintenance, highly numerous army, that maitains quality by constant infighting for supremacy between each other (strongest survive — a biological selection to make sure only the best warriors make it), yet stays relatively monolithic, through the guidance of the council, without the grudges between elves, dwarfs and man. Skaven are omnipresent, a feature important for the guerulla warfare Slann and Old Ones were fighting in, as their war became essentially a war of attrition, and they became like the occupied country, as chaos beasts rolled in. Skaven hate the clans that sided with Nurgle, two civil wars have proven it, yet they diversify their own population by making, essentially, warrior castes that each excel in one thing. But, much like the lizardmen — they have lost their aim, like their god, and therefore exist as a part of a machine that keeps going by itself, an engine that works, but without a propeller, for example, much like lizardmen — without a purpose (except their prime directive — absorb everyone who is not them and destroy as long as they exist). Skaven’s affinity for magic, their ability to make biological weapons (Moulder) and their hunt for warpstone (destroy your enemy’s fuel source, and you destroy him) make them a perfect anti-Chaos army.

This is a neat interpretation. I like it

[Greymene]

Except for Skaven being mutated creatures of Chaos, and that The Horned Rat being a lesser Chaos God, and as already mentioned that the ratmen were only created thousands of years after the Old Ones disappeared. But except for that...well, in another fantasy univers ratpeople could be perfectly natural creatues (at least as natural as anything else created by these mysterious Old Ones) that hated the powers of darkness. Sure.

[The Cunning Fox (raZoleg)]

I thought about the time difference, but forgot to include it in the post. Just like an old relic, the Horned Rat could only have appeared later, as, for example, an experiment that went out of control of the Old Ones, but had time to fester and come into existence.

Just like the Alien ship, that was found many many years after the death of the Engineers in Alien, Horned Rat could have been spawned out of the facehugger egg, the egg being the mysterious stranger with the bell who was unleashed onto the world and wondered for many thousands of years to find the city big enough to turn into ratmen by the black rain; as for it being a “lesser god”, the Old Ones could have used the same magic, or at least the same plane, where the Chaos Gods exist to make him, so they have made their own prototype god, as they realised that the best thing to defeat them is something that could be like the Chaos Gods themselves. They have created a Grey Hoo scenario, but they have had the failsafe to stop it… however — they’ve disappeared.

There are many contraptions and beasts left in the jungle from when the Slaan defended themselves against Chaos Gods. Just like minefields of the old wars, this too is a relic of the creation of their masters. Aimlessly wondering about, until they could find a suitable place to “infect”.

As for power struggles and civil wars — Pestilens was directly corrupted by Nurgle in far away lands. They have fought the disease and only accepted it due to isolation, much like the Death Guard who were trapped in warp and fought as long as they could against it. It was not just political struggle.

Also, Pestilens was only included into the council AFTER they have directly threatened them with disease outbreak. The council had no choice, and even after they were accepted into the Skaven clans — they still could not maintain a steady membership and another civil war broke out.

[RedDevil99]

I mean it's a cool enough fanfiction if that's the way you want to take it. But if you're being serious then well...

[The Cunning Fox (raZoleg)]

Well, nobody knows where he came from, nor why Tileans (I presume) and their dwarf friends turned into murderous rats, nor why isn’t he an actual full Chaos God, it’s just an explanation I came up with.

[RedDevil99]

Oh well I can answer that for you, the Tileans and Dwarfs didn't turn into Skaven, they got eaten by Skaven. As for why he isn't "an actual full Chaos God"... well he is. Just not part of the Fab 4 because he's a newer deity and thus hasn't established a powerbase similar to that of Khorne, Tzeentch, Nurgle and Slaanesh. That being said he still is a full Chaos God as he has his own lores of magic, his own Greater Daemons, and eventually after WHF dies and is replaced by AoS takes his place as one of the big 4 after Slaanesh's disappearance.

[The Cunning Fox (raZoleg)]

It’s actually hinted that they turned into the Skaven as it says they had nowhere to leave from the city. Some have wondered off, but “disappeared”. Some were eaten, like the last bunch, that only kept their human appearance because of the magical relics in the city, that warded off the spells of the rain. Also, why bother with rain, if you could just attack from directly beneath the ground and eliminate the dwarfs, then come out of every crevice and Menace From Below the manthings ass without the dramatic bell-tower, yes-yes? The rain had perhaps other qualities, other than just being black. It is shown that it takes a heavy toll on everything, psychological as well.

If he is a Chaos God, what does he represent? Khorne is war, rage and violence, for example, and the Horned Rat represents being horny? We already got Slaanesh on that. Also, why isn’t he mentioned anywhere outside of the WH Fantasy, but everything else, including Old Ones, Slaan, other Chaos Gods — is. Why don’t others worship him? Why doesn’t he have cultists outside of Skavenblight and their Under-Empire? He’s got his set of daemons, but like I said before — could be just what old ones created, a copy of a Chaos God, or their own design (with all the “bells and whistles”), just like Slaanesh was created accidentally out of a massive cocaine-and-hookers Eldar orgy, this could be a test tube god, made under controlled circumstances.

Originally posted by Zav:

Originally posted by razoleg (The Cunning Fox):

…and a deity they have manufactured to create the biological weapon — Skaven. Skaven transmute warmbloods into a minimum-maintenance, highly numerous army, that maitains quality by constant infighting for supremacy between each other (strongest survive — a biological selection to make sure only the best warriors make it), yet stays relatively monolithic, through the guidance of the council, without the grudges between elves, dwarfs and man. Skaven are omnipresent, a feature important for the guerulla warfare Slann and Old Ones were fighting in, as their war became essentially a war of attrition, and they became like the occupied country, as chaos beasts rolled in. Skaven hate the clans that sided with Nurgle, two civil wars have proven it, yet they diversify their own population by making, essentially, warrior castes that each excel in one thing. But, much like the lizardmen — they have lost their aim, like their god, and therefore exist as a part of a machine that keeps going by itself, an engine that works, but without a propeller, for example, much like lizardmen — without a purpose (except their prime directive — absorb everyone who is not them and destroy as long as they exist). Skaven’s affinity for magic, their ability to make biological weapons (Moulder) and their hunt for warpstone (destroy your enemy’s fuel source, and you destroy him) make them a perfect anti-Chaos army.

This is a neat interpretation. I like it

Thanks, man.

[ArchAnge1LT]

Horned rat is renegeade smaller Chaos god with all the deductions we can make or possibly a joint secret project of Tzeentch and Nurgle, who knows... (Nurgle alone would not be capable of this i think).

As for Old ones creating a chaos god. Well, i dont think they get along with Chaos well, huehue. Creating 'chaos' is kinda opposite of what Old one mission was. They wanted structured flourishing world, Chaos kinda is polar opposite, uncontrollable, parasitic bond with the world, it leeches from, transforms, mutates, and create all kinds of misery to establish even more dominance.

[Aleera]

Originally posted by razoleg (The Cunning Fox):

If he is a Chaos God, what does he represent? Khorne is war, rage and violence, for example, and the Horned Rat represents being horny? We already got Slaanesh on that. Also, why isn’t he mentioned anywhere outside of the WH Fantasy, but everything else, including Old Ones, Slaan, other Chaos Gods — is. Why don’t others worship him? Why doesn’t he have cultists outside of Skavenblight and their Under-Empire? He’s got his set of daemons, but like I said before — could be just what old ones created, a copy of a Chaos God, or their own design (with all the “bells and whistles”), just like Slaanesh was created accidentally out of a massive cocaine-and-hookers Eldar orgy, this could be a test tube god, made under controlled circumstances.

Thanks, man.

There's some hints and speculation that the horned rat is actually the chaos god Zuvassin. A minor god.
His realm being the realm of undoing. To undermine what others have accomplished.
Which fit the Skaven to the very core.

Like Hastut (Chaos dwarves) is the god of greed.

[The Cunning Fox (raZoleg)]

"Undoing" sounds like decay, so it's like Nurgle. This is redundant.

[Aleera]

Originally posted by razoleg (The Cunning Fox):

"Undoing" sounds like decay, so it's like Nurgle. This is redundant.

Decay is natural, Undoing is ruinination. Purposefully breaking things others made.
Nurgle is a perversion of the cycle of life.
Zuvassin is basicly jealousy, if I can't have it NOONE CAN.

[The Cunning Fox (raZoleg)]

But... it's not like Nurgle is making everyone die of old age, he is purposeful as well. Zuvassin might be so, but Horned Rat has no agenda, except to convert everything into a sea of the exact same nanobots. He is the Grey Goo, his simplicity in goals and his mystery of origins, as well as his seclusion from the rest of the unholy deities betrays his purpose.

If there was a theme you could sort of associate with The Horned Rat it would be conversion. Not exactly ruination or perversion, as the rats die of diseases and have a surprisingly strict structure and caste system, so they are not immune to decay and resist it just as much as others.

Also, notice how sneaky the rats are, and so is their god. He is the infiltrator, with a secret agenda, and this agenda is the undoing of the ruinous powers.

The simplicity, structure and secrecy of Skaven makes sense to me as Old Ones saw that Saurus, Skinks and Slann and other races they have created were being defeated by the Chaos, also due to the fact that they have been too complex to create and maintain, yet Chaos beasties were numerous and seemingly never-ending.

[Aleera]

There's 0 motivation for Skaven to convert others into themselves.
They can with the dreaded thirteen spell. But why? Breeding is better.
A new skaven is a new threat to their own position.

Because I doubt Tzeentch is about conversion either, but being near one of his pink or blue horros as they explode can make you become a chaos spawn.
Nurgle has a "zombie" plague.
All chaos gods try to get people to convert to their way, because that means more people that feed them emotion.

No, Skaven are not about conversion or the grey goo. I mean, look at how Skaven get ahead in station. You kill the person above you and take his position. Everything about Skaven is undoing what others do, so they can get better.

Yes there's overlap between the chaos gods. Even the top 4 have overlap.
A crazed champion of Khorne, that sheds blood, taking scalps, is actually revering Nurgle and Slaanesh as well.
Nurgle because of the dead body, from wich fungi, pestilence etc will grow. As well as the dopamine rush, his delight in bloodshed. Slaanesh gets of on that.
Only Tzeentch won't, but he's the polar opposite of Khorne.

So, no the horned one (he has 13 names btw, 1 might be Zuvassin. Only the council knows) is not a construct of the old ones. It's the anti-plan in all essence, the very opposite of what the old ones desire.

[The Cunning Fox (raZoleg)]

Yet Skaven started from the conversion, by occupying a city and changing its inhabitants and the bell tower. This is a hint -- they have not built a tower themselves, they have not started off in their own city -- they captured and mutated the inhabitants of the another one.

As for his names, those names could very well be the names of the engineers that have created him before -- nowhere is it hinted that it's other gods or deities. God Emperor of Mankind also had many names, yet he is known by more of an adjective, just like the Horned Rat, his name is a description, rather than an actual name.

Overlapping in some areas is not exactly being the essence of that particular area. Decay and undoing are nearly the same. Even for the slain enemy, that was killed in the name of Khorne, decay is the next step not part of the same step (especially if he was obliterated and there is nothing left to decay, for which Khorne does not care how he dies, whether he leaves something for Nurgle or not).

The reason Skaven get ahead by killing each other is because it's a warrior order -- only the strong survive. Should the weak survive also, they will not be able to overpower the other races and their ways of life, you see, and it would end up in either a stalemate or destruction of Skaven, instead of conversion. Just like in a debate, for example, you have to bring the best points to the front and discard the weak ones, to be able to overpower the will of your opponent.

They also cannot exist without fighting and consumption -- they die of starvation otherwise. Their lifespans are short too, so as to make sure they self-destruct after their task is completed, like the decomposers in nature (voles, for example, break down organic matter in the fields and live only a few months). Literally they have to consume and convert to survive, just like viruses and nanobots cannot replicate without absorbing matter and making more of themselves, and cannot survive in a stalemate or without expansion (well.. viruses can, but that's not the point here).

Everything tells me that they are, in essence, a "scary presumably-bad-guys-but-really-good-guys, to keep even the scarier bad guys away and thus save the universe", like in the old classic works:

“Who are you then?"
"I am part of that power which eternally wills evil and eternally works good.”

― Johann Wolfgang von Goethe, Faust