While it would make an excellent dinosaur name, the SCM, is alas, a muscle.
Sternocleidomastoid: It's a muscle that attaches at your sternum (sterno), clavicle (cleido) and your mastoid process (which is behind your ear), which governs your's head's ability to turn, bring ear to shoulder (side bend), and a few other paired actions. Why do I bring this mammoth named muscle up? Because it is one of the muscles that can be a serious problem in musicians, especially those of the violin/viola persuasion and flutists.
Forward head posture, which I mentioned on Friday, can be a huge pain (in the neck) but additionally can create muscular imbalance, wherein the muscles in the back of the neck (splenius) can get extremely overworked and overpower the smaller muscles of the neck, such as the SCM. In addition, the SCM can get overworked on its own if a musician perpetually leans the head to one side in order to play, like this famous dude on the right. Now that you know that the SCM affects the ability to turn one's head, as well as bring ear to shoulder (lateral flexion/extension), you can see why musicians sometimes have pretty serious neck pain, especially if they excessive clamp while playing a string instrument, overturn their head to the left or right (flute players, hello!), or if they don't take any preliminary steps to keep the head on straight (literally). If you think about it, an asymmetrical head position starts at a young age for many of us (violin lessons at 6) and for many hours a week, enhances that tightness and restriction on the left side. (Or right, if you're a flute player). Simply put, we have to keep the soft tissues of our neck healthy, because we're engaging in a practice that isn't really great for a vulnerable area of the spine. I like this video, as well as the practices I mentioned on Friday as a way to combat neck crises.