I've been doing some pet replacements mods for fun.
Some of them are: A shiba inu and a chihuahua.
I want to change the default cat now, but I don't own a cat with my current character, so I can't test.
I need a save file of a character that has a cat in the farm.

You'll generally have better luck asking things like this on the Stardew modding forum, but in this case, I think all you need to do is edit your save file (eg. Notepad) and modify...

<catPerson>false</catPerson>

to...

<catPerson>true</catPerson>

I've had a problem with Arcanum and I was offered multiple save files, throught the subforum. Now if I compare it with this...
Sadly this forum is very dead.
Even ConcernedApe stopped posting here, the creator of this game.
A small amount of people bought Gog version.
I'll post in the official/steam forum, 'cause I'm sure I'll be helped there, where the forum doesn't smell like corpses. :3

I did, a4plz, I've edited the save file with notepad++. But the game still acted as if my pet were a dog (for example, the portrait in the inventory window is of a dog). And that's not what I want, I don't want a dog that just looks/sounds like a cat. I want to edit the portrait of the dog and cat, not only the sprites.
But I don't need help anymore, someone at the official forum helped me.

Good to know! :3 Sorry, I just wanted to confirm whether that worked or not, so I don't repeat any bad advice.