Here's the title-bearer:
He's about 5", 8 cm from wing tip to wing tip. The reasons I call him a mutant are a little more obvious in this top view:
For completeness, here's the bottom view:
It was my first time seeing one of these. I had seen two other moths of this size -- a Luna moth many years ago, and a Polyphemus a few years ago while I was running in a 24 hour relay. Turns out they're all in the same family, the Saturniidae.
He was extremely patient with my photography and my wife going in and out of the sliding doors. Stayed put for several hours.
Like other members of this family, the adults don't eat. Go back to the top view -- no mouth parts. The adults try to reproduce, the females lay their eggs, and a few days later they die. Their main period of life is as a caterpillar. I've been calling this a male because it seems (the top link and further checking around the net) that the females are a more straightforwardly yellow. It is the males who have fair amounts of purple/brown.