While it is not known to evolve into or from any other Pokémon, it has an alternate form, which it changes into by using the Prison Bottle. Its default form, Hoopa Confined, will then become the Psychic/DarkHoopa Unbound. Hoopa Unbound in the party will revert into Hoopa Confined if three days have passed since changing to Hoopa Unbound. Other circumstances may also cause it to revert.
Hoopa Confined was officially revealed in the February 2015 issue of CoroCoro magazine. Hoopa Unbound was officially revealed in the May 2015 issue of CoroCoro magazine.
Hoopa Confined is a light purple Pokémon with a vaguely humanoid appearance. It has green eyes with yellow sclerae, a yellow ring marking on its forehead, a small, pink dot under each eye, and dark pink hair tied into a short ponytail. On either side of its head is a dark gray horn that curves slightly upward. Its body has a dark pink, V-shaped marking on the chest and a small, golden spike on each shoulder. Its arms are not attached to its body and instead float next to it; each arm has a small golden spike similar to those on its shoulders. It has mitten-like hands and dark pink rings around each wrist. In place of legs, it has a wispy tail. Hoopa has three gold rings: one large one around its lower body and one hanging freely on each of its horns.
Hoopa Unbound is a large, bipedal Pokémon with six arms. It is mostly light blue with purple fur and hair. The ponytail on its head is larger and bushier and held in place by two golden rings. Its hair now covers its chin, where it forms a pointed beard, and the back of its neck. The horns are less curved and have spiky purple hair around the bases. There are three gold rings on each horn: two around the base and one around the middle. It has yellow eyes white sclerae, a pointed, dark gray nose, a small hole inside of the ring-marking on its forehead, and prominent fangs in the back of its lower jaw. Similar to Hoopa Confined, its arms are disconnected and float next to its body. Instead of a small, gold spike the arms now have large, dark gray spikes and appear from holes along the sides of its body. Each arm has three fingers with dark gray claws, a gold ring around the wrist, and a gold ring around the base. Several tufts of purple fur surround its torso and a gold ring encircles a hole in the center of its chest. Another large gold ring encircles its waist and there are small tufts of fur poking out from underneath it. There is a purple stripe that widens at the knee down each leg and two gold rings around each ankle. The tips of its feet are gold and curl upward. Additionally, this form has a tail with a purple stripe along the top and an arrow-shaped tip. Hoopa Unbound is tied with Mega Gyarados as the tallest Dark-type.
The rings on Hoopa's body can warp space and allow it transport items by passing them through its hoop. Hoopa Unbound is said to be Hoopa's true form and is said to have the power to bend dimensions. This allows it to seize anything in the world. As seen in the anime, either form can travel through its rings and Hoopa Unbound has the ability to control and summon powerful Legendary Pokémon. Hoopa Confined is the only known Pokémon that can learn Hyperspace Hole, while Hoopa Unbound has its own signature move, Hyperspace Fury.
Hoopa Confined may have been inspired by common depictions of djinn, particularly those with wisp-like tails. Its having rings around its wrists could have been inspired by the djinn in the story of Aladdin.
Hoopa Unbound may also have been inspired by djinn, while its six arms may be drawn from the Hindu deities known as Shakti, who were rumored to grow multiple arms. It may also be based on the Gegenees, who were a tribe of six-armed giants who fought the Argonauts in Greek mythology. Its tail and horns may also derive from common depictions of demons.
Both forms of Hoopa could also be partly based on the legend of Nezha, a deity worshipped in Taoism and Chinese folk religion and commonly referred to as a trickster deity and a worldly protector. Very much like Hoopa Unbound, Nezha is depicted holding rings and is sometimes depicted with multiple arms. Its ability to use its hoops to transport things may be based on portals.
Its relationship with the Prison Bottle is similar to the djinn's with its lamp in traditional Arabian stories, since its full power is sealed within its respective vessel. Hoopa's only being able to maintain its Unbound state for three days parallels the limit to the number of wishes a djinni can grant to its current master. In both Formes, its Shiny coloration reflects on the Arabian origins of djinn.
Hoopa is derived from hoop. It may also be derived from hoopla (something loud, attention-grabbing, and possibly misleading).