Goomy is a Pokémon that resembles a slug or a slime with beady black eyes. The lower half of its body is light purple, while the upper half is a paler shade of purple. Along the dividing line of the two colors are five large, green spots. The front part of this line is its mouth. On top of its head are two pairs of antenna-like horns; the hind pair is longer than the front pair. These sensitive horns are used to check its surroundings.
Goomy is considered the weakest Dragon-type Pokémon. Its body is mostly water and is covered by a germ-laden mucous membrane. This slimy layer causes physical attacks to slide off harmlessly. However, it becomes unable to breathe and will die if its body dries out. Because of this, it stays in damp, shady areas. Goomy's horns have powerful sensors that allow it to locate any sign of the enemy, allowing it to survive any kind of danger. As seen in the anime, its diet consists of leaves.
In Hisui, when Goomy evolves into Sliggoo, it gains an additional Steel-type along with a metallic spiral shell due to its mucus having a reaction to the iron-rich waters of the region.
Goomy seems to be based on soft-bodied gastropods, such as slugs and nudibranch; its typing and eventual dragon design in evolution may be a reference to the "blue dragon" sea slug. It may also be based upon the concept of living slime in popular culture. Goomy also bears some resemblance to the prehistoric wiwaxia.
Goomy may be a combination of goo and either gummy or slimy.
Numera may be a combination of 滑る numeru (to be slippery) and ぬらぬら nuranura (onomatopoetic word for slimy).