You do not get a Red Junglefowl laying a 2000s definition of a chicken egg. You get a Red Junglefowl laying an egg with a mutation that that "Red Junglefowl" will pass on.
Every generation the Red Junglefowl becomes closer to the 2000s definition of a chicken.
It wasn't a "mutant" in the sense that one Red Junglefowl was born to create the chicken egg what we know as a 2000s definition of a chicken.