Toro, the mini-Human Torch on The Invaders, was born immune to fire. He joined the circus as a fire eater, and then discovered he could set himself on fire. And fly. We learn later that he was not a mutant, though–he was an Inhuman.
His birth parents worked for Professor Horton, who invented Human Torch, but they died and he was adopted by circus owners.
Oddly enough, they spend a lot of time establishing his connection to Professor Horton. But since he’s an Inhuman, there’s really no reason for it.