Quilladin is a bipedal, mammalian Pokémon with some plant-like features. It is largely covered in an armor-like shell similar to that of a chestnut. The armor is green in coloration with two large spines jutting out like ears. These spines and its pointed tail have red tips, and there are banded segments on the back of the shell. The lower half of the body is brown, and it has a three-pointed patch of fur decorating the front. Quilladin's stubby limbs are brown, but the top surfaces of the arms are protected by green, banded armor. It has three white, clawed digits on its hands and two on its feet. The face is exposed and unarmored, being light brown in color. It has a small, pink nose and a pointed snout, with a three-pointed patch of brown fur extending out like hair. Brown triangles mark its cheeks. Quilladin also has wide eyes, buck teeth, and usually wears a cheerful expression.
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.