What sorts of animals would make good pets for life in a space station, or aboard a ship? Smaller is better, obviously, but that still leaves a lot of options. How many resources would, say, a dog or cat consume? Air, food, and water come to mind right off the bat.
Actually, air, alone, brings not only added stress to the carbon dioxide scrubbers aboard, but also the micro-filtering, since furry animals shed — not only hair, but dander as well. You'd also have to import pet food food, and they'd consume water. Then you'd have to deal with their "business".
True, if we're talking about small dogs and cats, then we're not seeing very much CO2 to deal with, or stress to the micro-filters, but still, it's something. Pets aren't free to keep, even here on Earth, if you're actually taking good care of them, so some added expense is not a surprise. But, air is precious and finite aboard ships, and the recycling of it on stations represents a not-insignificant part of their operating budget. On a space station, and even a ship, you'd probably have to pay a special fee or tax for having a pet.
The same holds for, say, a rabbit, guinea pig, a hamster, or hedgehog. And there'd likely be ordinances against keeping rodents like mice or rats as pets, since they are disease vectors all across space. Frankly, mammals are a hassle.
So, how about a bird, like a parrot? Well, many of the same issues with mammals crop up, really. Air, water, waste products. Expense.
Fish? They do consume O2, but not as much as warm-blooded animals, and it has to be made available as free oxygen in the water, anyway. Keeping up with their tanks, though, means using a lot of that very precious resource, all of which would have to be cleaned and processed.
Reptiles, then? Snakes, lizards, turtles? Heat rocks or warming lights aside, they probably have a smaller consumption footprint than do mammals of equivalent size. I had a semi-aquatic turtle once, though, and let me tell you, he took tons of work, and dirtied a lot of water. Maybe a desert tortoise, then, since they don't use much H2O? Okay, I can see an argument for one of those as a pet. Sure, they consume O2, and you still have to clean up after them, which means the station has to spend money to deal with their feces, but frankly there wouldn’t that much, compared to many other animals. Let's put desert tortoises on the Maybe List for now. But while a lot people love reptiles, let's be honest...most people don't. I'm unconvinced they'd be popular.
How about something a bit more exotic, like spiders, or insects? They aren't going to use up gobs of oxygen or water, so, that's pretty good. Some of them would be banned right off the bat, of course, like cockroaches and centipedes; but almost any kind of tiny critter can become a pest, if handled carelessly. And most people rather hate insects and arachnids, so there's that.
My judgment, then: the perfect pet might be the simple house plant: it gives off oxygen and naturally scrubs CO2; give it some full-spectrum light, a little water (depending upon type), and maybe some pruning now and then. Rather than a cat, why not grow a tiger plant? What about basil? It will add life and fresh air to your quarters, to say nothing of its culinary gifts!
Plants may not cuddle up to you in the middle of the night, but on the upside, they don't leave squishy presents for you to step in, either.