Sceptile is a reptilian, bipedal Pokémon. Its neck is somewhat long, and it has two crests on its head. It has semicircular, yellow eyes with red rims. Its lower jaw and a belt-like band across its waist are also red. Along its back, it has two lines running down the middle and two rows of yellow nodules that are described as seeds. Its tail is shaped similar to that of a palm tree's branch. Both of its long arms have two sharp, elongated leaves and three claws. In Generation III, Leaf Blade was its signature move.
Sceptile is a jungle Pokémon. The seeds on its back contain nutrients, which are said to be able to revitalize trees. It is capable of leaping freely from branch to branch. While it has the ability to slice down thick trees, it is known to care lovingly for the plants in the forest. This Pokémon regulates its body temperature by basking in the sunlight.
As Mega Sceptile, its head crests become more pointed with a round portion missing from the middle. Red marking surrounds its eyes, which become smaller and more narrow. The yellow seeds down its back grow larger. The last pair of seeds turn red, while the pair above it turns orange. Red stripes encircle the base of its tail and run across its lower belly. The leaves on its wrists become more pointed, and the upper ones have red tips and half-circles cut out of the middles. Covering its chest and shoulders are plates of leaves similar to those that form its tail. Mega Sceptile's tail grows longer and develops a red stinger at the tip.
Mega Sceptile can cut off a portion of its tail to fire it like a missile at an opponent. Its lost tail will continue regenerating as long as it still has the energy required for Mega Evolution.
Appearance-wise, this Pokémon resembles Dilophosaurus, a dinosaur. The orbs of nutrients on its back may be based on the reproductive spores of fern species or based on the fact that many leguminous plants have nitrogen stored in nodes on their roots. The leaves on the tail in combination with the brightly-colored orbs on its back also create a strong resemblance to both the yew tree and various members of the genus Podocarpus. The Treecko line's appearance and mannerisms are also strongly derived from leaf-tailed geckos. Sceptile's green skin and red jaw seem to take inspiration from male green anoles, whose dewlaps inflate and turn red to show dominance.
Its ability to launch its tail while in its Mega Evolved form may be a reference to autotomy, a method of self-defense in which many animals, like reptiles, shed off their own appendages, which they can later regenerate. It may also be a reference to rocket launchers, with its tree-like tail also resembling a rocket.
Sceptile may be a combination of scepter, referring to its status as the "forest king"; scissors, referring to the blades on its wrists; or Sceptridium, a genus of grape-ferns; and reptile.
Jukain may derive from 樹海 jukai (abundant leafage; dense woodland).