Muscle is the name of a special type of mutant that roams the surface of Rome in packs. The name was given by the Swiss Guard of the New Vatican. The Swiss Guards believe that it's derived from humans, since it shows some signs of human appearance.
Muscles physically resemble a big white mass of flesh similar to a oversized muscle, with six pairs of legs and arms, resembling a cluster of six human beings stuck together in a single creature. The creature has no eyes and no ears, but has a flattened nose, like a bat. Muscles are unable to see or hear anything, but have a great sense of smell, being able to smell a human being from several metres away. Their method for hunting is to unleash a pheromone, described as good smelling, that hypnotises humans, forcing them to stand still, while the creature slowly approaches them. The only way that the Swiss Guard knows to counter them is by covering themselves in what they call "Dead Juice", which is a special type of liquid "perfume" made from corpses that makes Muscles unable to smell the wearer. The Swiss Guard isn't sure about what happens to a Muscles' prey, but at least three guardsmen disappeared without a trace after being hypnotised by them.