We know what “fat cat” means but my research does not tell me why the phrase contains the word “cat”. The person who first coined the phrase could have said “fat dog”. But it doesn’t sound as good does it? I think the major reason why the word “cat” came into the phrase is because it rhymes with the word “fat” and the idea was to incorporate an animal into the saying. It just sounds good and also conjures up a lazy individual who happens to have become rich not through endeavour and hard work but perhaps because the person inherited a large sum of money and was born with a silver spoon in his mouth. Domestic cats are perceived as lazy so the word “cat” fits in well.
It is said on Wikipedia that the term “fat cat” was first used by a writer for the Baltimore Sun, Frank Kent. He wrote an essay entitled “Fat Cats and Free Rides” which was published in the American Mercury in June 1928. He describes the sort of person a fat cat is in more detail in the essay. Essentially it describes a wealthy person who wants to dabble in politics and have some influence because making money no longer satisfies him or her. The term normally (always?) applies to men without being sexist. I have never seen an illustration of a fat cat woman.
You could argue that the term is a reflection of a male dominated society. It is still a man’s world some people would argue. In fact, you could say with a lot of confidence that it is still man’s world, which, for me, is not a good thing. It should be a world managed equally by both sexes.
The interesting aspect of the phrase “fat cat” is, as mentioned, the use of the word “cat”. If Mr Kent had used the word “dog” it would have sounded more derogatory because there is this very unfair reference to dogs in derogatory sayings about people. Two phrases come to mind immediately: “dog days” and “sick as a dog” or “you wouldn’t treat a dog like that”. The dog is using a very negative way in those sayings. And therefore the connotations of the word “dog” does not fit nicely into the meaning of the phrase “fat cats”. And of course it doesn’t rhyme. That’s why I argue Mr Kent made up this well-known phrase.
From a cat advocate’s point of view it is disappointing because the phrase creates a negative image of people and arguably it rubs off a little bit onto domestic cats. They receive some negative fallout from this derogatory phrase. Another cat saying that has negative connotations is “crazy cat lady”. That is bad for women and cats. It was almost certainly first used by a man. A sexist remark.
Conclusion: The “cat” in “fat cat” is there because the saying rhymes and because “dog” does not conjure up the required image in one’s imagination.