I would think that would be the abs, all the arm muscles and some on legs

Like many other strokes Butterfly uses a variety of strokes. The dolphin kick alone uses your multiple ab muscles, such as the rectus abdominus and erector spinea. Also you must point your toes when u kick so, your lower legs and ankle flexibility come into play. Also, your hip flexor and quads are used with the kick. For your upper body you are using your shoulder muscles (Deltoids)and your back muscles (latisumuss dorsi). Your triceps are also important when you have your catch while pulling. Overall, you pretty much use all your muscles in your body. Hope this answers the question.