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Decidueye did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Decidueye is a tall, avian Pokémon that resembles an owl. The upper portion of its face is black with a thin, orange mask-like marking encircling its eyes. Its eyes are reddish-orange and it has hooked black beak. The feathers covering its wings and back are brown, while its body is primarily white. The three feathers at the tip of each wing are structured similar to fingers and have lighter tips. The backs of its wings and torso are dotted with many white spots; the inside of each wing has a line of upside-down, reddish-orange triangles across the top. It has long legs, feet with two-toes facing forward and one backward, and short black talons. It tail consists of three long, green leaves.
A leafy hood covers its head and shoulders, and partially covers its face. The hood is smoothly draped across the shoulders and back, but has uneven edges. Around the head, the hood forms many pointed projections that extend over its face. A single white feather with a red base extends from the top of its hood. In the center of its chest is a reddish-orange x-shaped formed by dead leaves with two thin green vines extending from it. Pulling on the vines closes the hood further over Decidueye's face.
While this Pokémon is cool and cautious by nature, it begins to panic if surprised. It can hide from opponents by masking its presence, allowing it to attack unaware foes. It takes only a tenth of a second for it pluck, aim, and fire an arrow quill from its wings. It is not only quick, but also accurate, capable of striking targets over half a mile away.