I personally would not eat dog (unless perhaps there was nothing else available), because of their carnivorous nature, which means that heavy metals, chemical pollutants and so on will be present in the flesh in higher concentrations than in herbivore flesh. For the same reason I would avoid cat, rat and so on.
Horses however eat grass, and would be comparatively safe. Someone also mentioned mad cow disease which would be another advantage - at least on a theoretical level - but seeing as hardly nobody ever died of eating mad cows that risk isn't really worth taking into account.
Horses are packed full of nutriciously rich, succulent, scrumptious meat. Just look at a horse's rump - it's positively begging to be cut up into giant delicious steaks and cooked to perfection. There's NO reason whatsoever not to eat them, or at least no reason that makes any sense to a rational mind anyway.