Quilladin is a bipedal, mammalian Pokémon with some plantlike features. Most of its body is covered by a green, armor-like shell similar to a chestnut. The shell covers everything except its face, lower body and legs, and the undersides of its arms. The visible parts of its body are dark brown, except for its lighter face. The face has dark, oval eyes, a small, pink nose on a pointed snout, a dark brown, sideways triangle on each cheek, rodent-like incisors, and a three-pointed patch of dark brown fur on its forehead. On either side of its head is a large, ear-like spike with reddish tips. Its tail also has a reddish tip. Its shell is banded over its arms and on its back, and the fur on its lower body creates a three-pointed pattern on its front. Quilladin has three white, clawed digits on its hands and two on its feet.
Quilladin is not aggressive and never starts fights, although it is known to exercise its lower body by running into other Quilladin. When it is forced to defend itself, it relies on its shell to deflect attacks, before counterattacking with its sharp quills. The shell of Quilladin is both sturdy and heavy, which naturally strengthens the lower body of Quilladin. A Quilladin's tackle is powerful enough to send a trailer flying. Due to this body, Quilladin is immune to pain, not even violent collisions will faze it.
