Sliggoo is 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 and other organs.
Sliggoo excretes and coats its body with a sticky mucous that can dissolve anything. Because it lacks teeth, it sprays this liquid onto prey and slurps them up once they are dissolved. 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 seems 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 the Lou Carcolh, a mythical serpent/mollusk creature from French folklore.
It may also be based upon the concept of "living slime" in popular culture.
Sliggoo may be a combination of slick, slither, slippery, slimy, or slug, and goo.
Numeil is from 滑る numeru (to be slippery) and snail.