This is the atomic mutant from The Day the World Ended. Here, instead of transforming into a lumpy and hideous creature who must hide his face from the rest of humanity, he has transformed into a giant, dynamic superhero primed to kick other mutants butts. Thus does nuclear radiation act as a positive force in the world.
The original mutant is a Paul Blaisdell creature and features the pointed ears that most of his monsters sported. The details of the movie have faded in my memory. I’m pretty sure that the mutant creeped me out. Certainly its first appearance did. The whole monster didn’t appear. Just its three fingered, black furred hand reaching in from off screen to pick up a dropped hankerchief.