There's a Huge Exception From Demon Slayer's Most Important Ground Rule
Every demon was once a human, you say?
In Demon Slayer, the hit series that conquered the hearts even of those who were not really into anime, demons are eerie creatures that were once humans but then were turned into vampire-like carnivores who devour on people and have lost their humanity.
Basically, every single demon in Demon Slayer was a human once, even though most of them have lost their "human" memories and their former selves. But there is one big exception.
Remember the Demon Slayer Corps' loveliest member, Lady Tamayo's cat? His name is Chachamaru, and he even made an appearance in the third season. Unlike all the other demons (well, except Tanjiro's sister Nezuko), Chachamaru is a "good" demon who manages to survive without killing and eating humans.
Chachamaru can be compared to the Cullen family in Twilight, if we may: instead of devouring on humans, he sustains on donated blood. Besides, he violates another ground rule of the Demon Slayer universe: unlike other demons, he wasn't created by Muzan, but rather by Lady Tamayo.
She studies demon transformation because she is an expert doctor; Lady Tamayo was one of the first people Tanjiro turned to when he tried to heal his sister Nezuko.
Chachamaru was most likely Lady Tamayo's first step in her experiments. The cat proves to be an exception to two major rules in the universe: he was never a human, and Muzan was not the one who created him.
While the demon cat is more of a lovely sidekick in the anime and not a particularly important character, he still remains Demon Slayer's big mystery. After all, not every character breaks not one but two basic rules of the demonic universe!