Sliggoo is a Pokémon that resembles a snail. The lower half of its body is light purple, while the upper half is a paler purple. It has featureless, green eyes, a single, tiny eyelash over each eye, and two pale green spots on each cheek. It has nubby, underdeveloped arms. On top of its head are two pairs of antenna-like horns; the front horns are small nubs, while the hind horns are considerably longer. Its gelatinous body forms a spiral shell-like structure on its back. Contained within the shell are its brain, heart, and other organs.
Sliggoo excretes and coats its body with sticky mucus that can dissolve anything. Because it lacks teeth, it sprays this liquid onto prey and slurps them up once they are dissolved. Since its slime can melt through anything, predators stay far away from it. Its eyes have devolved, causing it to go blind. This makes it difficult for it to tell allies from prey and it will sometimes attempt to melt its companions. Its four horns act as a radar system and are used to detect sounds and smells. Sliggoo only cares about food and will flee from its enemies whenever possible.
Hisuian Sliggoo has droopy, teal eyes that imply its melancholic personality, and carries a metallic spiral shell that lies parallel to the ground. Unlike its Kalosian form, it has one teal spot on each cheek rather than two. It tucks almost its entire body inside this shell, and will only let out its head, neck, and arms to peek outside.
When Hisuian Sliggoo wants to move quickly, it will tuck its entire body into its shell, position the shell vertically to the ground, and roll itself away. It can also use its shell to float on water. It's theorized that the shell was created as a result of Hisuian Sliggoo's slime reacting with the iron-rich waters of Hisui. Both forms of Sliggoo can typically be found within marshlands or near bodies of water.
In Hisui, Sliggoo has a dual-type Steel/Dragonregional form. It evolves from Goomy starting at level 40 and evolves into Hisuian Goodra starting at level 50 during rain or fog in the overworld. All Goomy in Hisui will evolve into Hisuian Sliggoo regardless of their game of origin.
Sliggoo's Pokédex entry in Moon contains an error. Its slime is referred to as mucous instead of mucus. Mucus is a noun and a synonym for slime. Mucous is an adjective, and describes something related to mucus. This error was fixed in Ultra Moon.
Sliggoo seems to be based on soft bodied gastropods such as slugs, nudibranch, and semi-slugs. Its snail origins as well as its Dragon typing and elongated body shape may mean it could also be based on Lou Carcolh, a mythical serpent/mollusk creature from French folklore. It may also be based upon the concept of "living slime" in popular culture.
Hisuian Sliggoo is likely based on a snail. Its metallic shell in particular may be based on the scaly-foot gastropod, a species of deep-sea snail unique for its shell, which consists of iron sulfide compounds.
Sliggoo may be a combination of slick, slither, slippery, slimy, or slug, and goo.
The name may also be derived from the Irish word 'sliogán' - meaning shell - in reference to the shell on its back.
Numeil may be a combination of 滑る numeru (to be slippery) and snail.