Sneasel is a bipedal mustelidPokémon that resembles both a cat and a weasel. It is primarily bluish-black with three bright red feathers for its tail and a single one on its left ear. The ear feather is shorter on the female than on the male. The other ear is short and pointed, and it has narrow red eyes. On its forehead and chest are oval, yellow gem-like protrusions. It has red eyes with black eyelash-like markings on the edges. It has long limbs with two large, white retractable claws on its hands and feet. It is perfectly capable of using its claws to do substantial damage. Its claws are also used for climbing trees in boreal or sub-alpine forests where it lives. It can also bring out its claws to startle foes that attacked it.
Sneasel is a smart, sneaky, and notoriously vicious Pokémon, and blends well into the night in order to ambush their prey. Sneasel feeds on Eggs, such as those of Pidgey, that it steals from unattended nests. It uses its claws to poke a hole in the Egg before slurping the contents out. It has been known to form pairs to hunt: one Sneasel will lure the parents away from their nest, while the other steals their Eggs. Although the Sneasel pair works together, they will also fight over their hunt among themselves. As a result of its Egg-based diet, Sneasel are despised by Breeders to the point where they try to drive away or eradicate the former. Sneasel also preys on Alolan Sandshrew, but the Sandshrew's skin is so hard that it can cause Sneasel's claws to snap off.
In Hisui, Sneasel has a similar appearance to its Johtonian counterpart. It has a white body with purple colorations on its face, chest, shoulders, and forelimbs. The feathers on its tail and ear are light purple; like Johtonian Sneasel, the ear feather is shorter on the female than on the male. It has rounder eyes than its Johtonian form, although it shares the latter's gem-like protrusions, as well as the large claws on its hands and feet. The claws are tipped with poison, and are purple on its forelimbs and white on its feet.
Hisuian Sneasel's sturdy and curved claws allow it to climb and travel through the walls of precipitous cliffs where it lives. The venom from the tips of its claws is capable of invading the nervous system of any prey it scratches. It has been observed that Hisuian Sneasel lacks the spitefulness of its Johtonian counterpart, and it has been theorized that differences in their environments account for the differing personalities. As a result, Hisuian Sneasel evolves into Sneasler instead of Weavile.
Sneasel's most predominant role is as one of Silver's Pokémon. He first appeared in Who Gives a Hoothoot?. It is his first Pokémon as he was the Pokémon that he used during his training under the Masked Man, and the only one seen with him in flashbacks of his childhood prior to being kidnapped. He is a dexterous and crafty fighter that fully utilizes his speed and attacks of both the Dark type and the Ice type. After many years, he evolved into a Weavile in Lance's hidden Whirl Islands chamber.
Sneasel has been depicted with three variants in colour. In Pokémon Gold and Silver, Sneasel had a bronze coloration while its tail feathers were a dark cyan; while in Pokémon XD: Gale of Darkness, Sneasel had a light purple body. The third color variant is its current bluish black color which has been consistently depicted in the main series games from Pokémon Crystal onwards.
Sneasel may be based on the kamaitachi, a type of yōkai often represented as weasel demons with sickles on their front legs and sometimes understood as the cause of mysterious cuts on the skin, especially in cold weather. It also resembles black cats. It may also draw inspiration from the viverrids, weasel-like mammals with half-retractable claws that are sometimes mistaken for cats; its "feathers" perhaps represent the dorsal crest seen on some species. These feathers could also represent the wind itself, the key element of kamaitachi, as feathers are often associated with the wind, because they allow birds to move in the air and create gusts. Its eye markings are similar to that of ones in Egyptian style, possibly Horus's.
Sneasel is a combination of sneaky and weasel. It may also derive from sneeze, an action associated with having a cold or being cold, or sneesl (Scots for to begin to rain or snow) — both of which refer to its Ice type.
Nyula may be a combination of 潜入 sennyū (sneaking in) and 鼬 yū (weasel, an alternate reading of itachi). Alternatively, it may be a combination of new, ニャー nya (the sound a cat makes), and 野良 nora (stray, as in a stray cat, as strays are usually violent and tricky).