The Mouse King can use his action to polymorph into a Medium black-haired human woman with dark skin and copper eyes or a Small white-haired halfling man with pale skin, dark eyes and a long moustache, a Medium mouse-humanoid hybrid, or back into his true Small mouse form. His statistics, other than his size, are the same in each form with the exception that only his hybrid and true forms retain his climbing speed. Any equipment he is wearing or carrying transforms with him.