Fantasy specializes in animal sidekicks. Dragons, unicorns, white stags. It’s almost more likely to have an animal sidekick than not, and even if not, there’s usually some kind of critter in the background at least. You’re on a planet, there’s an ecosystem.
Science fiction is more sparing with pets. Pets are something of a luxury, right? You wouldn’t take them into deep space if they weren’t useful.
Personally, I think science fiction could use more cool animals. The sky isn’t even the limit with scifi. They can be any shape, any size, any ability. They could be telepathic or breathe freezing gas. They could be big enough to ride around inside or tiny enough to go in your bloodstream.
A lot of the imaginological niche of pets gets filled by robots. If you’ve got a cute little not-quite sentient sidekick, it could just as well be an adorable robot as an adorable kitty. Some of the rest is aliens: if you’ve got a cool biological idea, why not make a whole alien species instead of just a pet? But there is still definitely room for furry or scaly friends, because when it comes down to it, there’s nothing quite like a cat.
You could just bring Earth pets into space. I think humanity might well bring cats on spaceships for the same reason they brought them on sailing ships: for the mice. Dogs have countless jobs to do on Earth and could do space jobs too. After all, a dog was the first cosmonaut. I think monkeys could probably handle zero-g pretty well.
We can also imagine enhanced Earth animals. Giving animals increased intelligence or size would be interesting. But, remember, they cease to be pets at a certain level of intelligence and start to be more like fellow persons. Does your fictional society have a standard? Or did they forget to make one, so that your cat is smarter than you but can’t vote?
Last are the truly alien animals. They may be alien life humans discovered and tamed, or have been domesticated by aliens. What kind of pet might a very different kind of life form tame?
One of my favorite books involving scifi pets is The Icarus Hunt, by Timothy Zahn. An alien character, Ixil, has two ferretlike “outriders” who can be sent out exploring and report back telepathically. Also they’re cute.
Honor Harrington also has an animal sidekick, Nimitz the treecat. Though honestly, reading the wiki (I didn’t get far in the books) I’m not sure he really counts as only an animal.
And who could forget the cat in Alien?
What are your favorite science fiction books including animals?