At the risk of sounding completely unhinged, a trope that continually fascinates me is the portrayal of spaying and neutering in animal xenofiction.
Obviously I'm mainly talking about animal xenofiction centering on dogs and cats, because getting fixed doesn't really come up for wild animals. And a lot of times it doesn't even come up even then, because stories about talking cats and dogs tend to be relegated to children's fiction, where the audience isn't going to care about it, authors might not be comfortable broaching the topic, and the Babies Ever After trope is a tried and true happy ending that, as a bonus, allows for possible sequels about the main characters' kids.
Almost all the examples I've seen where it does come up are cat stories: the Cutter in Warriors, Windflower in Tailchaser's Song, and the Hermanns in Felidae. And in all three of those examples it's played as unambiguously negative. Fear of getting fixed is part of what drives Rusty/Firepaw into the woods in Warriors, and meeting his neutered childhood friend is what confirms to him that he made the right decision. Windflower is a horrifying (arguably kinda transphobic? it was written in the 80s, your mileage may vary) cautionary tale of what happens to cats that get too close to humans and lose their "wild" side. Felidae takes the most neutral stance of these examples, but the Hermanns being neutered is a point of ridicule, and they're portrayed as pathetic toadies to the much stronger (unfixed, obviously) Kong.
And it's such an interesting moral quandary to me because spaying and neutering cats is, unambiguously, a good thing! Not just for humans who don't want to deal with unwanted kittens, not just for the environment that gets ravaged by strays over-hunting the local wildlife, but for the cats themselves too: unfixed cats are more aggressive, more unsociable, and more prone to illnesses and infections. But I can understand why fictional cats being happy about getting the snip might be a hard sell. Even the stories most devoted to portraying animal characters as nonhuman still need to have some level of anthropomorphism in order to engage the human audience. Readers put themselves in the shoes of the main characters, so they're not going to see themselves as the humans, they're going to see themselves as the cats, and because forced sterilization is a horrific violation when done to humans, it's probably going to be come off that way when it happens to animal characters.
Fiction tends to take it a little far, though. In Tailchaser's Song, getting neutered basically feminizes Windflower, but another character heavily implied to be spayed is shown to be completely uninterested in doing anything but walking from her food bowl to the couch and back again. Warriors similarly shows fixed cats like Smudge to be lazy and indolent and uninterested in doing anything but sleeping and eating. In both cases it's almost a lotus eater situation, where cats that rely fully on humans will have all their needs met and live in luxury and pampering, at the cost of being forcibly robbed of an "inner spark". Which is patently untrue in real life, as I'm sure many cat owners would know: spaying and neutering doesn't stop your cat from wanting to hunt and fight and play, that's why TNR is an imperfect harm reduction method that doesn't prevent strays from damaging local bird populations.
Anyway, what brought this on is that I found an example of cat-focused fiction that completely averts this trope. It's not mainstream like Warriors; it's a still fairly new webcomic called The Turning of Seasons that focuses on a cast of stray cats. At one point on a recent page, one of the cats mentions that "the Silver Cages" are back, implied to be TNR traps, and it's revealed through dialogue that fixed cats ("moon toms" and "moon mollies") have an established position in their cat society, as fighters, no less. It's the first time I've seen a cat story where getting fixed is portrayed as potentially desirable to the cats themselves, with main character Red's only reservation being that she might want kittens herself before she makes such a big decision. Instead of being this horrific atrocity because Humans Are Cthulhu, it's actually folded into the worldbuilding for how this stray cat society operates.
It's an interesting departure from a trope I've seen more than a few times, as an avid fan of animal fantasy growing up. I'd be fascinated to see if it develops in future stories, especially indie stories created by people who grew up with the ones that established the trope in the first place.