Muscled means having large, easy-to-see muscles. Writers often put another word in front: a heavily muscled boxer, a finely muscled horse. It tells you how a body looks. It does not say the muscles are stiff or that the training went too far — that would be musclebound. The word is more at home in writing (sport, stories, animals) than in casual chat, where people often just say muscular. It is built from muscle plus -ed, like a body that has muscles.