A cat is a small, indestructible, furry automaton created by God to give you love and companionship when things fuck up your shit of a life. Nah, just kidding, nobody would love you, especially creatures who have at least some amount of self-dignity. Contrary to popular belief, cats secretly conspire every possible moment on how to murder their owners and escape to the wilderness. Cats have the ability to break spacetime by at times expanding to fill all available space. (see Catsplosion)
Cats are accurately depicted in Dwarf Fortress.
Dungeons and Dragons
An amusing consequence of the 3rd edition D&D system is that a domestic housecat is capable of demolishing first level commoners in combat, doing a point of lethal damage with a single bite or claw attack and being able to make two claw attacks and one bite attack in a single round. First level human commoners generally have between 1 and 5 hit points. Further more, they are harder to hit on account of their tiny size and higher dex (which together give them an AC of 14). The size bonus also boosts their attack, and because they have weapon finesses, their dex does too (which together give them +4 to hit). However, longtime cat owners could testify that this is arguably realistic. For example, my ungrateful cat just scarred my hand, nyoro~n.
Cats in Warhammer Fantasy
Like in old folklore stories, Queen Neferata, the first Vampire and head of the Lahmian Bloodline, likes to turn her underlings (usually female) into cats to sneak into the homes of men (or women) of high standing. The shapeshifting ends, and the Vampire seduces their target leading to them either being their mysterious mistress (in all applicationsof the word) or their mysterious relative/spouse nobody had heard of. Either way, Neferata gains another pawn in her plan for world domination. In old Lahmian models cats often appeared alongside the Vampire.
Cats in warhammer 40k
It was confirmed in one of the Ciaphas Cain books that humans still keep cats as pets. And when you consider how many rats and shit must be in those hive cities, it's not exactly surprising. They would be quite at home on a ship of the Imperial Navy, but sadly, many might also just be food for hive world inhabitants and feral worlds, anywhere where food is hard to find (much of the Imperium).
It's also fun to imagine what might happen if the AdMech were to find an old server bank containing all of humanity's old LOLcat photos... ah, wouldn't cybercats in red robes be awesome?
A cat found by an Ork, though, if not eaten by the Ork himself, would be quickly gobbled by Squigs. though if the cat possible had a underbite like some Persians and exotic short hairs it could be possible that the orks would be dumb enough to think of them as a fellow orks and recruit them too the waaaagh and with the orks brain power I think so therefor it must be true the cat would officially be a small furry ork.
The ruinous powers most definitely have cats with them; cats of Khorne would be angry psychopathic bags of claws that eat weak cultists for dinner (see Dex-Starr). eventually ascending to demon hood as to continue their crusade against bigger prey I mean who wouldn't want to see a space marine fight a demon cat prince. "Death to the false empurrer! fluff for the fluff god, mice for the mice throne!" also would probably murder the shit out of chakats for being a bunch of psychic Mary sues. ahhh doesn't the thought seeing chakats having their skin peeled off by the furry rage of khorne seem so soothing, the demonic house cats adorned with brass armor that blocks out the chakats cries for help, the pain induced spasms that are now visible, the thought of this chakat's skull being made into the second skull throne aka the skull toilet reserved only for chakats and all their subspecies.
Slaaneshi cats... you know what's going to happen, I don't even need to explain this. Cats of Nurgle would be flea-ridden mangy mongrel kittens, rubbing up to Loyalists and infecting them with Nurgle's love. possible they're cats that got infected with some kind of warp originating illness but papa Nurgle picked them up and took them to his garden to adopt them and so became his furriest demons yet as the go around the under-hives infecting mice as to make sure Nurgle's love is in every corner of the galaxy arriving at the garden to make sure new plaguebearers come to term with what they have become.
And finally, felines would likely fit in with Tzeentch's followers, what with the Bond villain vibe of the Change-lord himself. also the cats randomness would meld into tzeentch's randomness making them into greater daemon of fluffy insanity that tears up a solar governor's furniture while no one is looking and turning the man into a paranoid freak who thinks that there's a monster destroying his most prized possessions up until the cat literally starts scratching at the insides of the man's head making lose his sanity with every new scratch and every new nail piercing his sanity, soon all he hears is scratching and meowing as he's unable to sleep and drifts away and begins to hallucinate and begs to something anything for the scratching to go away and in this moment tzeentch appears in the man's lucid nightmares and says that all his possessions will be put back together and that scratching will stop but only if he vows loyalty to him and him alone, and in sheer desperation says yes and proceeds to fall to sleep on the floor and when he awakes all his furniture and lively possessions are rebuilt but at the cost of his eternal allegiance to tzeentch his fate and the fate of his worlds are now under the control of a mad god. and for his service the warp kitty gets a big ol' fish and told she or he's a good kitty and then the cat runs off fish in hand to do his lords bidding.