For Americans and, to a lesser extent, other English speaking people, eating horses is like eating pigs for Jews. It's a cultural taboo that is so thoroughly ingrained that the thought of eating a horse can elicit feelings of disgust. In this case, it's not because horses are viewed as "unclean" but because they historically were so much more valuable as draft animals. Additionally, cattle are so plentiful that there is no incentive to slaughter horses for meat