The purple pentapus is a small, five-legged cephalopod that leaves red marks when its tentacles are detached from a person's skin. The creature is difficult to remove with brute force once attached, but if one strokes its head, a gesture which the pentapus immensely appreciates, it will readily release its grip.
Team Avatar encountered this species in the city of Omashu while navigating through a sewer pipe. When Sokka crawled out of the sewer pipe, he had several pentapuses clinging to his face. Upon removal several spots were left over, and the team used it as an excuse for being around during the night, saying they were escorting their sick friend with pentapox, a fictional disease. The distinctive suction marks left by these animals were later used as a way to lead the citizens of Omashu outside the captured city's gates. The pentapuses' suction marks fooled the guards into believing that all the inhabitants were infected with pentapox.
The purple pentapus is a five-eyed, five-tentacled, octopus-like creature that lives in the sewers of Omashu and is roughly the size of a human fist. Using the small suction cups on its tentacles, it latches onto targets, though it is apparently harmless. These tentacles are used to grip the sides of sewers in order to eat algae and other food.
Penta (πέντε) is a Greek numerical prefix for "five", and pus is derived from the Greek pous (πούς), which means "leg". The name thus refers to the five limbs of the animal, much like how the octopus' name comes from it having eight limbs.
In the video game The Burning Earth, the pluralization of "pentapus" was noted to be "pentapii", versus the more conventional pluralization of "pentapuses".