How do you know that "ancient people" never thought of animals as having feelings? Seems an extraordinarily broad claim given all the cultures and ages that encompass "ancient people"
I mean even my own currently-alive ancestors (farmers, the majority of them) don't look at livestock as living, conscious beings. They obviously care for each animal and give them a swift and as-painless-as-possible butchering when the time comes, but they're not the bleeding hearts that many people these days are, they're mostly just vehicles for vital necessities.
I don't find it difficult to imagine my great-great-great-great-(fill in the appropriate number of greats here to qualify for ancient ancestors) grandmother butchering a pig and not really giving it a second thought at all, having grown up on a farm myself. Hell, my current grandma is probably much gentler and gives the pork a much better time than my ancient ancestors did
Because I read some of their writings? The concept of a "Pet" was foreign to them. Animals - even dogs, were there to work.
The concept of how an animal was feeling simply didn't occur to them. An animal was a tool, and you took good care of the tool, but not because of the tool, but because then it was more useful.
It was the 18th century when animal rights started becoming a thing, which exactly corresponds with the industrial age when things became less scarce and people could worry about animals, and not just themselves.
"Some of", indeed. The poster above is correct that your statements are over-broad.
There are hundreds of Roman monuments, inscriptions, and poems [celebrating dogs](https://thepetrifiedmuse.blog/2015/06/20/every-dog-has-his-d...) as companions and pets. There are medieval European graves that strongly suggest sentimentality towards the animals buried in them. There's this [lovely ninth-century Irish poem](https://www.poetryfoundation.org/poetrymagazine/poems/48267/...) about a cat. Someone with more knowledge in the area than I possess should comment on animal portraiture in ancient and early-modern China and Japan, but that was a thing as well.
I think the fairest thing to say is that people in every era - very much including today - instrumentalize some animals, and sentimentalize others.
You read some writing by some "ancient" people (post writing 'ancient it seems) and you're launching into broad sweeping generalisations about all "ancient people" ?
Wow.
Meanwhile, I've travelled a lot for work - mostly to odd corners of the world, and I've yet to meet people that didn't have stories about animals and animal behaviours.
Famously, for example, Australian Aboriginal Dreamtime stories are largely about animals, Tiddalick the Frog, Emu and the Jabiru, etc.
They encapsulate the place of animals in the environment, and as hunter gatherers attuned to where next years meal will come from, attention is paid to breeding and caring for the young so that there are full grown adults to breed again and to eat.
Rightly or wrongly the stories are about the imagined feelings of animals, the things that make them happy and plentiful, the bad things that cause numbers to dwindle.
Various people had various relationships with various animals, not as "pets" but as other beings in the world.
