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I've been getting into Wing Commander again lately and reading Niven's Known Universe, and Andre Norton is a life long favourite. All of these have something in common, they all have an intelligent, spacefaring race, or races, that are described as "catlike" or even specifically feline in nature. So I've started wondering who was the first to use this "space-cat" idea and in what work?

In my opinion it is The Cats of Ulthar by HP Lovecraft (c) 1920. Wikipedia summarizes as:

"The Cats of Ulthar" is a short story written by American fantasy author H. P. Lovecraft in June 1920. In the tale, an unnamed narrator relates the story of how a law forbidding the killing of cats came to be in a town called Ulthar. As the narrative goes, the city is home to an old couple who enjoy capturing and killing the townspeople's cats. When a caravan of wanderers passes through the city, the kitten of an orphan (Menes) traveling with the band disappears. Upon hearing of the couple's violent acts towards cats, Menes invokes a prayer before leaving town that causes the local felines to swarm the cat-killers' house and devour them. Upon witnessing the result, the local politicians pass a law forbidding the killing of cats.

The sentience here is inferred from the action: they can act collectively, communicate (through prayer and each other), seek revenge, and coordinate a plan: all this is abstract level thought.

This will be a theme in Lovecraftian horror. These cats of Ulthar reappear in The Dream-Quest of Unknown Kadath, in the alternative reality of the Dreamlands where they converse, war, and more. Here the cats display very human like qualities and are only different from human actors in their eldritch qualities of being feline and other. I don't know if the travel between earth and the Dreamlands is by definition space travel as the Dreamlands are more or less an alternative dimension.

This is shaky SF at best but H.P. Lovecraft in The Dream-Quest of Unknown Kadath had sentient Earth cats traveling to the moon where they were menaced by evil cats from Saturn. It was written late in his career and not published until 1943.

In short, rhe story definitely included sentient alien cats in space but it isn't usually classified as science fiction.

IMHO the latest possible date for the first "catlike" or "feline" intelligent aliens in science fiction would be 1952, the year when The Mixed Men AKA Mission to the Stars by A. E. Van Vogt was published http://www.isfdb.org/cgi-bin/title.cgi?229641.

In that novel a planet of a star in the Large Magellanic Cloud is inhabited by intelligent beings with centaur-like body shapes and catlike features. An alien classification system similar to that in the Hospital Station series by James White or the Lensman series by E.E. Smith is used and the aliens are classified as CC meaning catlike centaur.

The aliens in that planet aren't advanced enough to have interstellar travel, but since the Earth Empire apparently rules millions of planets back in the Milky Way Galaxy and aliens with the CC classification are apparently common, it is possible that many space travelling catlike aliens have been encountered in history.

The Mixed Men AKA Mission to the Stars (1952) contained several stories published earlier as well as new material written for the novel. Thus it is possible that the catlike centaur aliens go back to an story published earlier, possibly "The Storm" in the October 1943 issue of Astounding Science Fiction http://www.isfdb.org/cgi-bin/title.cgi?467362.

Another intelligent catlike being is the antihero or antagonist in the first science fiction story published by A.E. Vogt, "Black Destroyer", in the July, 1939 issue of Astounding Science-Fiction http://www.isfdb.org/cgi-bin/title.cgi?416043. Coeurl doesn't come from a space travelling civilization, but tries to steal space travel technology from humans and create a space travelling civilization of his species. "black destroyer" was revised and included in the novel The Voyage of the Space Beagle (1950) http://www.isfdb.org/cgi-bin/title.cgi?84014

So in the works of one science fiction writer, A.E. Van Vogt, his earliest catlike intelligent alien beings would date to either 1952, 1943, or 1939. However, they do not totally match the original question since their cultures are not yet spacefaring a the times of the stories.

1947: "The Cataaaaa" (aka "The Cataaaa"), a short story by A. E. van Vogt, first published in Fantasy Book, Vol. 1 No. 1, 1947, which is available at the Internet Archive; the reprint in Marvel Science Stories, November 1950 is also available at the Internet Archive.

The first-person narrator is a biology professor. The alien space cat is allowing itself to be exhibited as a circus freak:

We had already seen the fat woman and the human skeleton, but Silkey took us back and told us his life history with them. How he had found them, and helped them to their present fame. He was a little verbose, so on occasion I had to hurry him along. But finally we came to a small tent within the tent, over the closed canvas entrance of which was painted simply, "THE CAT". I had noticed it before, and the chatter of the barker who stood in front of it had already aroused my curiosity:

"The cat . . . come in and see the cat. Folks, this is no ordinary event, but the thrill of a lifetime. Never before has such an animal as this been seen in a circus. A biological phenomenon that has amazed scientists all over the country . . . Folks, this is special. Tickets are twenty-five cents, but if you're not satisfied you can get your money back. That's right. That's what I said. You can get your money back merely by stepping up and asking for it . . ."

[. . . .]

The animal that sat in an armchair on the dais was about five feet long and quite slender. It had a cat's head and vestiges of fur. It looked like an exaggerated version of the walkey-talkey animals in comic books.

At that point resemblance to normalcy ended.

It was alien. It was not a cat at all. I recognized that instantly. The structure was all wrong. It took me a moment to identify the radical variations.

The head! High foreheaded it was, and not low and receding. The face was smooth and almost hairless. It had character and strength, and intelligence. The body was well balanced on long, straight legs. The arms were smooth, ending in short but unmistakable fingers, surmounted by thin, sharp claws.

But it was the eyes that were really different. They looked normal enough, slightly slanted, properly lidded, about the same size as the eyes of human beings. But they danced. They shifted twice, even three times as swiftly as human eyes. Their balanced movement at such a high speed indicated vision that could read photographically reduced print across a room. What sharp, what incredibly sharp images that brain must see.

The narrator has a private meeting with the cat, who tells him what it's doing on Earth:

The creature nodded. It said after a moment, "I won't tell you what energy my brain develops. It would probably frighten you, but it isn't all intelligence. I am a student on a tour of the galaxy, what might be called a post-graduate tour. Now, we have certain rules—" It stopped. "You opened your mouth. Did you wish to say something?"

I felt dumb, overwhelmed. Then, weakly, "You said galaxy."

"That is correct."

B-but wouldn't that take years?" My brain was reaching out, striving to grasp, to understand.

"My tour will last about a thousand of your years," said the cat.

[. . . .]

"You came in a spaceship?"

The cat looked at me thoughtfully. "No," it replied slowly. "I use the energy in my brain."

"Eh! You came through space in your own body?"

"In a sense. One of these years human beings will make the initial discoveries about the rhythmic use of energy. It will be a dazzling moment for science."

"We have," I said, "made certain discoveries about our nervous systems and rhythm."

"The end of that road," was the answer, "is control of the powers of nature. I will say no more about that."