How frequently does the letter t appear in:
Q: Does the Horned Rat appear in WarHammer 40k? The Horned Rat is the God of all Skavens in WarHammer Battle, and is a minor Chaos God compared to the 4 main Chaos God. 
WarHammer Battle and WarHammer 40k are two separate universes, but they have a very similar mythology, with many gods in common. Though there is no skavens in space, has the Horned Rat ever been mentioned as a minor Chaos God in 40k (in a Codex, or a novel, or any official material) ?

A: TL;DR:


*

*There are more than just 4 Gods in the 40k universe

*There is a reference to a minor God called Malice.


First, there are more than just four Chaos Gods. There are four Major Chaos Gods, but there are a number of Minor Gods that exist in the warp as well. A great many consider Daemons to be Gods, but Daemons are created from the Chaos Gods powers to do their bidding. Really the question comes down to what do you accept as canon, since both Games Workshop and the Warhammer fanbase battle both with each other and themselves as to what is and is not canon.
Now we know the two universes are linked through the warp; but I do not think we know for sure whether Fantasy comes before 40k or not. It might actually follow or be concurrent... The chaos gods are always susceptible to shifts, especially in the minor arenas but I have never heard of the Horned Rat in 40k. But the fact that some Gods are prominent in both universes is key that it is possible to be in both.
I found a post that referenced him in 40k as the Horned Prince, but even if that were true the site labeled him as a Daemon prince. To my knowledge no Daemon Prince has ever found a way to become a Chaos God, minor or otherwise... Though it is possible for a Chaos God to use all of its power as a God to create a Daemon and thus transform from a God to a Daemon.
It does seem however that there is mention of a Minor God named Malice, The Lost God. He is named in a short story called Labyrinth by Richard Ford. I mention this because in my research I found references to The Great Horned Rat as Malal or Malice and malice, blight, etc. are part of his repertoire. There is also a confirmed Space Marine Chapter that was excommunicated when they turned to Chaos and began to follow this Minor Chaos God.

A: Nope
As you point out in your question, there are no skaven in space.
A chaos god gets all of its power from its worshipers, and there are no skaven to worship it so the Horned Rat just couldnt exist in 40k. And to my knowledge, there has been absolutely no mention of it in any of the army books or fluff.
Ive always assumed that the numberless tyranids were the 40k equivalent of skaven, since the burrowing rat men probably wouldnt translate to space very well.
In the fantasy universe at least, its arguable that the Horned Rat is the most powerful of all the Chaos gods as it undoubtedly has the most followers, theyre just to busy backstabbing each other to ever do anything. (Until the End Times anyway).

A: The 1989 compendium mentions that Ratlings sniper rifles have a skaven toxin available. The Skaven were also planned to be included in Warhammer 40k at some point, as mentionned by Jes Goodwin in The Gothic and the Eldritch, which includes a few sketches of 40k skaven (They were mostly WWI-inspired apparently, with a lot of chemical warfare). This never went anywhere though, and there are no mentions of the Horned Rat anywhere in Warhammer 40k.

A: NO
There are 4 Gods in the W40k universe.
Khorne
Slaneesh
Tzeentch
Nurgle 
There might me a minor daemon of Nurgle that could posses the same/similar characteristics as the Horned god. Pestilence is the domain of Nurgle.

user
The total number of "t"s in that passage is 251.
assistant