In Incarnate Forme, Thundurus is a blue, muscular humanoid Pokémon, which resembles a genie. It is a male-only species with no female counterpart. On its head is a single, long, sharp, purple horn. Thundurus has white, zigzagging and jutting styled hair, a spiky white mustache and two pointed blue ears on either side of its head. Its eyes are yellow with beady white irises and no visible pupils. The front of Thundurus's body is covered in various, irregularly shaped purple spots. Its broad chest consists of two joined, purple, rounded-diamond shaped pectorals and its shoulders, elbows and navel are adorned with raindrop like designs. It has two spotted muscular arms, with five fingers on each hand. Its lower body consists of a trailing cloud, with a thick, long, curling, purple, cable-like tail. Spanning the length of its tail, Thundurus has several ball-like structures. They are a dark grey color, and each has a long, sharp spike on one side.
In Therian Forme, Thundurus is a large, blue, sleek, bipedal snake or dragon-like Pokémon. The horn on its head is now thicker and extends out more, while its ears become more pointy and pinned back. Thundurus's hair remains the same; the mustache now takes on a more raised snout and fanged-maw-like appearance, with two curving wisps flowing back and past its ears. Its eyes remain yellow, but with larger irises which still lack visible pupils. its chest remains broad and unchanged, while its navel is bejeweled with a blue gem. Thundurus's arms have lost much of their muscular bulk, and are now slimmer. It has two paw-like hands, each covered in a trailing cloud-like fluff and with three claw-tipped fingers. Thundurus's purple lower half is much smaller than its upper body, and consist of two hind-legs and its tail. Its feet are elongated, with three downward curved toes and claws. Thundurus's tail also remains largely unchanged, although it is thicker at the base and tapers off and becomes thinner toward the tip.
Thundurus can shoot thunderbolts from its tail.