This is a chicken that was raised in a horse stable for a couple months and was just introduced to the rest of the flock. He is the only one that looks like this.
The picture as well as your "narrative" indicate you have a sick, very sick chicken. That's all. Without knowing the age, seeing a sample of its poop, knowing what it has likely eaten, the climate, your location, flock history as to how many have sickened and died this season, daily behavior of the flock, we can't even begin to guess what could be going on.