This is an odd request, but I've been having trouble finding the kind of games I'm looking for with itch.io's search system.

So you know the fairytales/goosebumps books/horror stories/etc. where a magic-user curses or transforms someone into something else? I'm trying to find games where that is a big part of the story, because I've played a couple of games like that in the past and they always ended up being really interesting both narratively and in implementation(there was one where you had to manage your time before the curse was complete that  I remember really liking). Does anyone know any good games like that?

The first one that comes to mind is an old game, "Bad Mojo"; in that game the player-character is transformed at the start into a cockroach.

None of these are full horror games, but some have some horror-like elements.  I'll update this post as I remember more.  These games should mostly be pretty good though, even if they aren't horror.

Majora's Mask has some of those elements, if you haven't played that.  The recent Prey reboot has some aspects of that, but it is sci-fi.  There's also a game based on Kafka's Metamorphosis, no idea if it is good though.

Momodora: Reverie Under the Moonlight has a point where that happens, but again, not strictly horror.

Bloodborne has the "curse of the beast" as a massive part of it's plot, where people turn into monsters.  Not a full-on horror game, but has a horror-like atmosphere.  Hard game but one of the best games of all time in my opinion .

Final Fantasy XIII heavily features a magic transformation element, but is not a horror game at all, and is not considered to be an especially good game, but some like it.