I don't know why this just occurred to me, but wouldn't the castle just be a huge bestiary if every student had a pet? They can only bring cats, toads, and owls, but we've also seen a dog, a phoenix, and a rat, so exceptions exist.

Let's assume there are 300-500 students. If only half bring pets, that's still about 200 animals roaming around. The owls would stay in their tower, but that would still leave dozens of cats and toads in the castle. Are they confined to their owner's dorms? Common rooms? Are they allowed to roam the school or leave grounds?

Where are the bloody litter boxes? Imagine the smell and the constant hissing and fighting. There would be shed fur covering every surface in every common room. How many cats are murdered every year because someone's roommate gets pissed off by constant nighttime yowling? How many toads get squashed when someone stumbles in the dark to go pee? There's no way in hell that Neville's toad survives for seven years. XD

Are there any fics that deal with this in a funny way? It sounds like a great oneshot waiting to happen. I can only remember reading one story from Trevor's perspective, and that was a long time ago.

 

What did you think house elves ate?

 

What do you think Filch is doing all day?

 

I think you're over estimating the number of people who have pets. The Weasleys for example only have one family owl shared between 9 people and Percy's owl until Sirius gifts Ron an owl. Hermione got a pet in third year but her parents are probably quite wealthy. The only other pets acknowledged in the books are Malfoy's eagle owl and Neville's toad.

My suspicion is that most people don't bother with owls at Hogwarts because the school provides owls, no one has toads because they're lame, leaving only cats which we don't see much of. Harry has Hedwig because Hagrid wanted to get Harry a gift and Hagrid likes animals.

 

Well, I wouldn't have guessed cat poop or cat hair, but now I'm wondering. It might explain a few things about house elves.

It's possible, but it's also possible that JKR left out the description of everybody else's pets to avoid narrative bloat. We didn't know Bulstrode had a cat until it was necessary for a plot point. What else don't we know? It's a silly question and doesn't matter in the end, but it could be comic fodder for a story, especially if all the animals are allowed to roam around the school.

 

We're confined to Harry's perspective, and after a point he stops caring about first years carrying around toads because they have trouble making friends or separation anxiety. Pottermore says Trevor wandered off into the lake to join the other amphibians, which I suppose could be literally true or euphemistic.
Additionally, I would imagine most non-first years would rather not have to keep up with an animal and probably discontinue bringing it.

 

I would go with their cooking materials.
Hogwarts is its own eco system.