I've read a few manga where a girl is turned into a slave or pet through things like training, medication, body modification, language and sense control devices, and brainwashing. Sometimes this issue is portrayed in society as being completely natural. For example:
Ane Inu
One of my favorites is that the author left the story unfinished. In his world, there are animals that look like humans, but walk around naked and on all fours in society. There is also a type of collar to control them. My favorite part is about a girl named Miho who wears that collar and loses her ability to speak and lives and is treated like a pet.
Oh! My Dog
It seems like animals are going extinct and an organization comes along and takes people, cuts off fingers, puts animal ears on them, erases their memories, and sells them as animals, and this is completely normal.
Love Pet 1×1
Unfortunately, I didn't read this translated, but it seems to me that some people in society are like pets and are treated like them.
Boku to Suzuna no Ita Natsu
In this one, which is also untranslated, the main character receives a pet as a gift that looks like a Human, but is a domestic animal.
Seeing Eye Dog
The main character is a girl who goes to a mansion for work, but faints and when she wakes up, her hands and feet are tied and she can't speak. She is supposed to continue her life as a dog for a blind boy.
Fest Works
An artist named Fest has a whole body of work on Human Livestock that I personally follow with enthusiasm.
These were some things I read and found interesting. If you know of any similar works, whether manga, manhwa, novels, anime, or games with this feature, I would appreciate it if you could tell me.