It is not known to evolve into or from any other Pokémon.
Pyukumuku is a marine Pokémon with an ovoid, black body. Its face has two beady, pink eyes and a white mouth shaped roughly like a star or Daimonji. This Pokémon is capable of spitting its organs out through its mouth. The white organs can take different shapes, such as a fist. Two lines of three pink spikes run down each side of its back. The middle spike of each line is about twice as long as the others are. A fluffy white puff forms its tail.
Pyukumuku tends to find a spot it likes and stay there, even returning to the same spot if moved. Once all the food in its favorite spot has been consumed, it will continue to stay there to the point of starvation. Its body is coated with a viscous fluid that allows it to stay on land for a week without drying out. If stepped on, it will expel its fist-like organs and particularly hates having its mouth or spikes touched. Tourists dislike Pyukumuku due to its appearance and lifestyle and often accidentally step on Pyukumuku. This creates a need for part time jobs on beaches chucking this Pokémon back into the sea. However, people in Alola have already taken to throwing the Pokémon back to the sea where it can find food when thin. The fluid that coats its body is beneficial to people as a moisturizer.  Stepping on a Pyukumuku will make it mad and proceed to smack its victim.
Pyukumuku appears as a Pokémon summoned from the Poké Ball. It uses Counter whenever an opponent gets close enough to it. Pyukumuku then waves "goodbye" before it leaves. It can also be picked up and thrown like an item.
This Pokémon was unavailable prior to Generation VII.
Pyukumuku may be based on a sea cucumber, some species of which expel their Cuvierian tubules (part of the sea cucumber's respiratory system) to deter would-be predators from eating them. Given its tail and "ears", it may also be inspired by the sea bunny, a type of sea slug that became popular in Japan in 2015 due to its cute appearance. It may also resemble Wiwaxia, a kind of ancient soft-bodied sea creature.
Pyukumuku may be a combination of puke, cucumber, and mucus.
Namakobushi may be a combination of 海鼠 namako (sea cucumber) and 拳 kobushi (fist).