Taste probably has a lot to do with it, but I think personability does, too. I think it's a little sickening to eat a dog because dogs are some of the most personable animals on the planet. Horses, as opposed to cows and deer and chickens, are extremely personable. A lot of it may be historical, too. The horse has probably been more useful in human civilization than any other animal, providing us transportation, "horsepower" for agriculture, many new sports, and other things, and much more. It doesn't seem natural to consider these animals food, too, when even now they are still prized for their beauty and utility.