It is not known to evolve into or from any other Pokémon.
Shuckle in its shell
Shuckle is a Pokémon that resembles a small turtle. Its body is yellow and appears soft. Until they are in use, Shuckle's limbs appear limp. It is encased in a very hard red shell that has many holes in it. The holes in its shell are rimmed with white. Rather than actively hunting and foraging for food like most other Pokémon, it collects Berries and stores them in its shell. The Berries inside are liquefied and fermented while they decompose by Shuckle's digestive juices, which it then consumes. Its digestive juices are also used to dissolve rocks so Shuckle can hide under them. It can be found in the mountains.
Multiple Shuckle made their main series debut in A Better Pill to Swallow, where an old man was using the Berry Juice that they make to create medicine. One of the Shuckle that he befriended was a purple Shuckle whose Berry Juice attracted Pokémon to anyone who drank it. Because of this, Team Rocket made it their mission to steal it.
Shuckle, the Mold Pokémon. When Shuckle places organic materials in its husk-like shell, the items are transformed into a unique juice. Shuckle are naturally shy and are most often found hiding beneath rocks.
Shuckle seems to be based on an endolith, an organism that lives inside porous rocks or animal shells. Given its category, the Mold Pokémon, Shuckle may be based on a fungus or mold variety of endolith. Its category could also allude to fungi such as cedar-apple rust, which bears a strong resemblance to Shuckle, or fungi-like slime molds.
Given its Bug typing, Shuckle may also draw inspiration from scale insects—small, shelled parasites that produce a sweet fluid called honeydew that attracts other insects. In particular, it may be based on the barnacle scale, with characteristics of actual barnacles. This species is part of the Coccidae family, also known as "tortoise scales", which may explain Shuckle's resemblance to a turtle or a tortoise. It could also be seen as a reference to the food fermentation process, with its Rock type alluding to the pottery used to store and ferment aliments in some cultures, and its Pokémon category referring to some fungi used in cultured foods.
Shuckle may also have been based on certain varieties of mollusk, as it shares some characteristics with them, including the shell, the muscular but limp-seeming feet, the use of acids to receive food or to break down obstacles, the liking for fermented foods, and living close to the water. Its worm-like aspect may also take inspiration from a nematode.
Shuckle may be a combination of shuck (a husk or pod) and barnacle or turtle.
Tsubotsubo may derive from 壷 tsubo (jar) and 富士壺 fujitsubo (barnacle).