Why is it culturally unacceptable to eat horse, but it is okay to eat a cow?
Thanks for asking! As you may know, your question is right at the heart of a number of huge, ongoing debates in ethics, food, and sociology. So I hope you don't mind if I offer a few sources for further reading but can't offer a definitive answer!
For starters, attitudes toward both horses and cows as food animals vary widely by country and culture. In some European countries horsemeat is perfectly common at butcher shops, and among certain groups in South Asia there are religious and even legal prohibitions against the slaughter and consumption of cows. This Forbes article presents a good quick survey of the history of American attitudes toward horses and horsemeat.
Of course, there's the larger discussion about the ethics of meat - you'll find a wide array of responses from carnivores, vegans and vegetarians, and animal rights activists. There's really not much to do but sit back and enjoy the debate.
Thank you for a very interesting question to spruce up the afternoon, and thank you for using AskOPL!