What is a hernia?
A hernia is a protrusion of an organ or tissue through an abnormal opening.
A perineal hernia occurs when fat, blood vessels or organs such as the bladder have the opportunity to leave the pelvic abdominal area and enter the area along side the muscles of the the tail, gluteals and anus.  Now those tissues
composing the hernia and normally  "in the abdomen" are escaping out under the skin of the rump.