Culturally, I'm pretty sure it has to do with which animals we view as pets/companions. It seems disturbing in the Western world to eat horses because we keep them as pets; you don't eat your pets. If we were to more broadly keep cows/chickens/pigs as companions rather than livestock, we as a whole culture would also find it disturbing to eat them. Hey, maybe that's where animal rights activists should focus their attention...a farm animal pets campaign. (I mean that sincerely; I eat meat cause it's easiest, but I do sympathize with the movement).