Applin is a small, worm-like Pokémon with a light green body and a dark green underside. Its head is round with two pink nostrils. Above its head are two large leaf-like eyestalks which house its two small black eyes. These eyestalks give the apple it lives in the appearance of having a stem with leaves. Applin also has two thin hairs on the tip of its tail, as well as two stubby forelimbs which are usually hidden inside its apple.
Applin is both the lightest and shortestDragon-type Pokémon, and also one of the shortest Grass-type Pokémon. As soon as it is born, it burrows into an apple that looks like a Leppa Berry, which serves as a home, food source, and protection against preying bird Pokémon. The apple is prevented from rotting thanks to Applin's body fluid, which also strengthens the fruit's rind. The flavor of the apple determines which evolution Applin will evolve into. If Applin runs out of apples to eat or if it is separated from its apple, its body will lose moisture, and it will become dehydrated and weak. Since Applin hides inside an apple, it is sometimes mistaken by the frugivorous Noibat for food. In Galar, it is a custom to give an Applin as a romantic gesture.
Applin appears to be based on the popular image of a worm burrowing into an apple; perhaps specifically the larvae of various moths, such as the geometer moth larvae (also known as the inchworm) and the codling moth larvae, who burrow into apples and feed on their cores, inhibiting growth and ripening them earlier. It may also be based on the mole lizard, which has two forelimbs and closely resembles an earthworm, but is actually a type of reptile related to snakes known as an amphisbaenia.