Trevenant is a ghostly Pokémon resembling a slender brown tree. Its shadowy black body is visible through gaps in the tree, and a single, red eye is visible through a hole near the top. There is a wavy, horn-like branch on either side of its head, both dotted with small, green leaves, and a smaller branch on its forehead. On top of its head is a cluster of green leaves. It has two arms with gnarled branches similar to its horns on the elbows. Around its wrists are cuffs of green leaves and it has three claw-like fingers on each hand. Instead of legs, it has six roots similar to a spider's legs.
Trevenant can control other trees by using its roots as a nervous system. It uses this ability to trap people who harm its forest and will curse people who cut down trees. However, it is very kind to forest-dwelling creatures and Pokémon that reside in its body. Trevenant hates Fire-type Pokémon, so lumberjacks usually bring them to repel it. Along with its pre-evolved form Phantump, Trevenant is the only known Pokémon capable of learning Forest's Curse.
Trevenant and its pre-evolution were designed by James Turner.
Trevenant is probably based on a kodama, the spirits of trees that sometimes inhabit old-aged trees. Its stripe-like indentation on its body is very similar to shimenawa, used to mark spiritual habitation, who are often seen tied around kodama. It is also similar to other types of tree and nature spirits and deities, such as dryads. Trevenant's fierce protection of forests may be a reference to hamadryad, or could perhaps be a nod to the concept of protected sacred groves. Its single eye may possibly be a reference to cyclopes or the mountain-dwelling yamawarawa. Its general body shape may have been based on the shape of the kanji 木, meaning wood or tree. Its Shiny coloration is based on the birch tree, known for its gray-and-white bark, with its leaves representing a tree in the autumn.