Appletun is a dragon-like Pokémon inhabiting a body reminiscent of apple pie. Its body is mostly green with a yellow underside and lower jaw, and has an orange spot on its chest that resembles an apple core. It has a piece of an apple on its head worn as a helmet. Its eyes usually droop down from under the apple helmet as though they were ears, but may be lifted up to reveal leaf-like eyestalks holding small beady eyes. It has black slits for nostrils on the tip of its muzzle. It has a large hump on its back with a several overlapping stripes that resemble an apple pie's crust. It has red and green leaves on the top of its pie-like back. It has short, bulky legs with paws, and a stout tail.
Appletun produces a nectar-like scent which attracts Bug-type Pokémon, which it preys on. Its body is caked in sweet nectar, which gets thicker when exposed to Gigantamax energy. The skin on is back is said to be tasty, and children used to eat it as a snack.
When Appletun gets exposed to Gigantamax energy, it changes into the shape of a giant apple with a green dragon-like Pokémon inside. Its leaf-like eyestalks stand up straight and it has the top of the apple on its head with yellow carvings on the apple resembling nostrils. Its previously stout tail is elongated and wraps around the base of the apple. The Pokémon has rounded cheeks and hangs its mouth open letting a red tongue hang out of its mouth. Three red clouds float around the neck.
The sweet nectar that covers its body increases in thickness, allowing it to absorb more damage than normal. It can shoot out nectar, enough to be able drown its opponents in the blast of nectar.
Appletun and Flapple are the only Pokémon to have physically identical Gigantamax forms, although they are still treated as different Pokémon as they have distinct Pokédex entries, G-Max Moves, and cries.
This is also the only instance of two different species of Pokémon that can be physically identical without the use of Transform, Imposter, or Illusion.
Flapple and Appletun having identical Gigantamax forms may be a reference to convergent evolution, which can lead to separate organisms independently developing identical characteristics and body plans.
Appletun may be derived from Appleton, a toponym that means "an orchard" or "place where apples grow". Appletun may also be a combination of apple and ton (referencing its bulk). It may also be a play on apple turnover or under.
Tarupple may be a combination of tart, 怠い tarui (sluggish), たっぷり tappuri (onomatopoetic word for plenty), and apple.