Exposure to just 800 rems would be deadly for us. If you want to kill a brony with radiation, it will take 67,500 rems to do the job
But there's more to a nuclear blast than radiation. There's heat.
Should the brony happen to be at the center of the nuclear bomb's target, it would find itself cooking at a temperature of well over 10 million degrees Celsius. Even 50 meters away from the epicenter of the blast, temperatures would reach about 10,000 degrees instantly. That's just not survivable, even for a brony.