I'm not even gonna be bothering with realistic cat designs, cats can have eye colours of any colour. Pelts can have any colour combinations (although, the colours are close to realism with the colours only being what's possible for cats to have on their pelts, so no purple hair or such. Although combinations of these colours I won't care for realism. If a ginger is gonna have black stripes, they'll have black stripes. Shades of these colours may from time to time stray from realism though, so yeah. ) and have all types of patterns. (Want a lightning bolt on their thigh? Heck yeah!)
I'm also not really touching on cat breeds, unless the cat is of housecat origin might they be some special breed, other then that breed traits can just, appear, and I all consider it being that these cats are all like, Perosekin cats? I wouldn't say they're a new species of cats, rather a very wacky regional breed spread all over the island. I can imagine Perosekin cats are seen as rather exotic to the rest of the world, but they can also be decently wild so don't go into the woods and just grab one okay?
Perosekin cats absolutely do get kept as pets, for awhile amongst clowders it was even a slur towards em to call them "domestic" but housecats never really understood what's meant to be offensive about it. So it's rather a slur that clowder cats will try to offend eachother with, but is ineffective against actual housecats which it's meant to be about.
Also! Clowder cats, besides Pinenians, kinda treat housecats like they're a whole different species for a good chunk of the series. They think these domesticated cats are unable of any real survival instincts and are considered weak, with their only power being sheer numbers and having humans (and sometimes dogs) as allies that make them a threat. This is seen as really cowardly though.
Of course, An Alder's Rise And Fall, heavily starts challenging these beliefs and widely changes how the Birchians view housecats, and they realize how the only thing that divides them is where they live and their culture.
I mentioned before how Pinenians aren't essentially racist towards housecats, and the reason is simply their very peace oriented beliefs and morals, and apart of this peace is not causing any problems about races, culture and so on. Because of this even, Pinenians are the Clowder most open to accepting immigrants, aka outsiders like housecats for example who wish to join. This has even been showcased by the fact how you're going to be more likely to encounter stranger names there (more human names and such) and cats may often be mixed too. (Ashstar has some housecat blood :I ironic)