P. mucosa is a non-venomous snake of the family Colubridae which is the largest family of snakes, the family is found in every continent with exception for Antarctica. P. mucosa is a large snake, individuals up to 2 meters occurs though adult size is usually between 1,5 and 1,9 meters. When adult this snake does not have natural enemies, and have been reported to even eat King Cobras.
About 28 centimetres (11 in) in length. Mostly crimson with black trailing edges to wings. underparts white with messy black marks. The breast and neck are black with white spots on the throat and white streaks on the breast. A black eyestripe stretches to the back of the neck and diffuses into the nape and upper back, with white streaks between the eye and neck. It has a red crown, with the forecrown and forehead black speckled with white in females. The eye is hidden among the black eyestripe. It has a fairly sized grey bill, which forms a blunt point. Juveniles are duller with less bold markings, with males having white spots on crown and females with few spots if not none.Hybrids can be either mostly red with orange or yellow (closer to Red-backed flameback) or mostly yellow with red or orange (closer to Black-rumped flameback)