It has been mentioned here already. Horse meat has about half the fat compared to beef. And beef already has very little of fat.
More importantly, horse is not considered for human consumption, therefore there is a whole lot of drugs allowed to be used throughout the lifetime of which nobody really knows how potentially harmful they could be to us, humans. Assuming we talk about a regular farm horses. Wild horses? Probably even better. Race horses? No thank you. Those things are fed with such potent cocktails that it is a miracle it lives as long as it does.